It's funny that people are angry at AT&T for locking a SIM but these same people have no problem with Apple's closed ecosystem or the requirement to have a mac device to watch product launches. Some of us use products from multiple vendors. A locked SIM is no big deal compared to some of Apple's restrictions.
I just switched back to an iPhone and some of these comments make we wonder about Apple fans. This site is much more fanboyish than when I visited back 6 years ago.
Key difference -- Apple stood on stage and said point blank, "Thanks to our new Apple SIM, you can switch between any of these carriers whenever you want and use one SIM card." AT&T obviously agreed to those conditions at some point because they're on the list of carriers that support the Apple SIM. Now, clearly, after the device has launched, AT&T has changed their mind.