The thing is that AT&T hasn't restricted adb install. If they had, then google would revoke their license to redistribute the google apps, and (hopefully) no-one would use the phone because it didn't have android market.
It's pretty simple - if you want Google's blessing, you can't restrict/lock down the adb protocol - which means you must allow installing applications via adb.
Now, the real problem would be if they did decide to remove all the Google apps and the Android branding, and went off and completely did their own thing. If that happened, I'm hoping the market would react and sales of the device would be poor. But then again, take a look at all the people who buy the locked down iPhones.... my optimism might be unwarranted in this case, unfortunately.