Why your iOS device with the AT&T SIM automatically join to an AT&T Wi-Fi Hotspot aka "attwifi - Passpoint" without any user settings.

AT&TのSIMを挿したiPhoneは、何故自動的にAT&TのPasspoint基地局Wi-Fi接続するのか

Let me jump to the conclusion.
The Carrier Bundles from AT&T includes a profile to connect AT&T Wi-Fi Hotspot (Passpoint).

I read some AT&T user's point in the forum like below;

How to Disable ATT Passpoint IPhone 8+ iOS 11.0.2
Yesterday I was at a HomeDepot and nothing was loading on my phone. It turned out I was on ATT-Passpoint WiFi. Not LTE. The phone won’t let me “forget” that ssid and when I toggle AutoJoin off it just switches back on.
I don’t care what security ATT passpoint uses I just don’t want to auto join any public WiFi system. Anyone know how to disable this?

https://forums.att.com/t5/Apple/How-to-Disable-ATT-Passpoint-IPhone-8-iOS-11-0-2/td-p/5287613

Forced to join attwifi since iOS 11!!!!!
According to AT&T's info here https://passpoint.attwifi.com/ an iPhone needs a Passpoint profile installed to automatically connect. However, my phone does not have any profile installed. I imagine that they updated the carrier settings to "force" configure the phone to work with Passpoint if WiFi Calling is enabled, perhaps to coincide with the iOS11 release.
https://forums.imore.com/ask-question/394777-forced-join-attwifi-since-ios-11-a.html


And yes, Here is the mecanism.
AT&T updated with thier "Carrie Bundles" with iOS11.
We can see what does it have.

The following link is the apple carrier bundle list:
https://itunes.apple.com/WebObjects/MZStore.woa/wa/com.apple.jingle.appserver.client.MZITunesClientCheck/version

It has the BundleURL list, iOS11.1 will see this URL.

		<key>ATT_US</key>
		<dict>
			--- snip --
			<key>11.1</key>
			<dict>
				<key>BuildVersion</key>
				<string>30.1</string>
				<key>BundleURL</key>
				<string>http://appldnld.apple.com/ios11.1/carrierbundles/091-43117-20171030-22774A0C-BD87-11E7-92F2-91FF17FB103A/ATT_US_iPhone.ipcc</string>
				<key>Digest</key>
				<data>
				+0Eoo93OM5FX2iXr6WiHNRQDHr0=
				</data>
			</dict>
			--- snip --- 
		</dict>

The .ipcc file is ZIP archive.

Here is file list.

Dutch.lproj           hi.lproj                     overrides_N56_N61.pri
English.lproj         hr.lproj                     overrides_N66_N71.der.pri
French.lproj          hu.lproj                     overrides_N66_N71.plist
German.lproj          id.lproj                     overrides_N69.plist
Info.plist            ko.lproj                     overrides_N69.pri
Italian.lproj         ms.lproj                     pl.lproj
Japanese.lproj        no.lproj                     profile.mobileconfig
Spanish.lproj         overrides_D101_D111.der.pri  pt.lproj
ar.lproj              overrides_D101_D111.plist    pt_PT.lproj
ca.lproj              overrides_D10_D11.der.pri    ro.lproj
carrier.plist         overrides_D10_D11.plist      ru.lproj
cs.lproj              overrides_D201_D211.der.pri  signatures
da.lproj              overrides_D201_D211.plist    sk.lproj
el.lproj              overrides_D20_D21.der.pri    sv.lproj
en_AU.lproj           overrides_D20_D21.plist      th.lproj
en_GB.lproj           overrides_D22.der.pri        tr.lproj
es_MX.lproj           overrides_D22.plist          uk.lproj
fi.lproj              overrides_D221.der.pri       version.plist
fr_CA.lproj           overrides_D221.plist         vi.lproj
global_setting_A.gri  overrides_N51_N53.plist      zh_CN.lproj
global_setting_B.gri  overrides_N51_N53.pri        zh_HK.lproj
he.lproj              overrides_N56_N61.plist      zh_TW.lproj


The profile.mobileconfig include the settings to autojoin the network of AT&T Wi-Fi Passpoint with the EAP-SIM/AKA authentication.
It does not have SSID as attwifi-Passpoint because it is Passpoint.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>PayloadContent</key>
	<array>
		<dict>
			<key>PayloadDescription</key>
			<string>Configures device restrictions.</string>
			<key>PayloadDisplayName</key>
			<string>Restrictions</string>
			<key>PayloadIdentifier</key>
			<string>com.apple.attwifi.restrictions</string>
			<key>PayloadOrganization</key>
			<string>Apple Inc</string>
			<key>PayloadType</key>
			<string>com.apple.applicationaccess</string>
			<key>PayloadUUID</key>
			<string>E1390FCE-911C-4B64-B352-8CE4B5E3A891</string>
			<key>PayloadVersion</key>
			<integer>1</integer>
			<key>allowAddingGameCenterFriends</key>
			<true/>
			<key>allowAppInstallation</key>
			<true/>
			<key>allowCamera</key>
			<true/>
			<key>allowExplicitContent</key>
			<true/>
			<key>allowGlobalBackgroundFetchWhenRoaming</key>
			<true/>
			<key>allowInAppPurchases</key>
			<true/>
			<key>allowMultiplayerGaming</key>
			<true/>
			<key>allowSafari</key>
			<true/>
			<key>allowScreenShot</key>
			<true/>
			<key>allowVideoConferencing</key>
			<true/>
			<key>allowVoiceDialing</key>
			<true/>
			<key>allowYouTube</key>
			<true/>
			<key>allowiTunes</key>
			<true/>
			<key>forceEncryptedBackup</key>
			<false/>
			<key>ratingApps</key>
			<integer>1000</integer>
			<key>ratingMovies</key>
			<integer>1000</integer>
			<key>ratingRegion</key>
			<string>us</string>
			<key>ratingTVShows</key>
			<integer>1000</integer>
			<key>safariAcceptCookies</key>
			<integer>2</integer>
			<key>safariAllowAutoFill</key>
			<true/>
			<key>safariAllowJavaScript</key>
			<true/>
			<key>safariAllowPopups</key>
			<true/>
			<key>safariForceFraudWarning</key>
			<false/>
		</dict>
		<dict>
			<key>EncryptionType</key>
			<string>None</string>
			<key>HIDDEN_NETWORK</key>
			<false/>
			<key>IsHotspot</key>
			<true/>
			<key>PayloadDescription</key>
			<string>Configures wireless connectivity settings.</string>
			<key>PayloadDisplayName</key>
			<string>Wi-Fi (attwifi)</string>
			<key>PayloadIdentifier</key>
			<string>com.apple.attwifi.wifi</string>
			<key>PayloadOrganization</key>
			<string>Apple Inc</string>
			<key>PayloadType</key>
			<string>com.apple.wifi.managed</string>
			<key>PayloadUUID</key>
			<string>9CB34BD4-FF73-4E49-8474-DA475B13E2C6</string>
			<key>PayloadVersion</key>
			<integer>1</integer>
			<key>SSID_STR</key>
			<string>attwifi</string>
		</dict>
		<dict>
			<key>AutoJoin</key>
			<true/>
			<key>DisplayedOperatorName</key>
			<string>AT&amp;T Wi-Fi Passpoint</string>
			<key>DomainName</key>
			<string>attwifi.com</string>
			<key>EAPClientConfiguration</key>
			<dict>
				<key>AcceptEAPTypes</key>
				<array>
					<integer>23</integer>
				</array>
				<key>EAPSIMAKAConservativePeer</key>
				<true/>
				<key>EAPSIMAKAPsudeonymIdentityLifetimeHours</key>
				<integer>24</integer>
			</dict>
			<key>EncryptionType</key>
			<string>WPA2</string>
			<key>IsHotspot</key>
			<true/>
			<key>MCCAndMNCs</key>
			<array>
				<string>310410</string>
			</array>
			<key>PayloadDescription</key>
			<string>Configures Wi-Fi settings</string>
			<key>PayloadDisplayName</key>
			<string>WiFi</string>
			<key>PayloadIdentifier</key>
			<string>com.apple.attwifi.com.apple.wifi.managed.C7F682EC-9305-4E07-A1AB-BEB24B1E5611</string>
			<key>PayloadType</key>
			<string>com.apple.wifi.managed</string>
			<key>PayloadUUID</key>
			<string>865A5DF1-1AE7-4659-85D8-B686E29BB1AE</string>
			<key>PayloadVersion</key>
			<integer>1</integer>
			<key>ProxyType</key>
			<string>None</string>
			<key>ServiceProviderRoamingEnabled</key>
			<true/>
		</dict>
	</array>
	<key>PayloadDescription</key>
	<string>Auto-joins the attwifi Wi-Fi network</string>
	<key>PayloadDisplayName</key>
	<string>attwifi</string>
	<key>PayloadIdentifier</key>
	<string>com.apple.attwifi</string>
	<key>PayloadOrganization</key>
	<string>Apple Inc</string>
	<key>PayloadRemovalDisallowed</key>
	<false/>
	<key>PayloadType</key>
	<string>Configuration</string>
	<key>PayloadUUID</key>
	<string>7156353A-F9C3-42AE-A781-2DD301ACBBD0</string>
	<key>PayloadVersion</key>
	<integer>1</integer>
</dict>
</plist>