And that's the trick: AT&T implemented throttling just after all the dedicated iPhone users upgraded to an iPhone 4S and locked themselves in for another two years.
You didn't have to lock in. If you paid full price for the iPhone 4s, then when you first plugged it to iTunes, you got a "congratulations, your phone has been unlocked" message, and the phone stopped being carrier locked. This worked since iPhone 4s release day, before the 'official' unlocked phone shipped.