Is it because of the unlockers? YES
Why can't they require a signup online? Good question since I did that for my first cell phone about 5 years ago. If you want my opinion, AT&T got scared about unlockers (thanks a lot, guys) and went way overboard by padlocking the doors and putting a 7'2" ninja-trained bouncer a the door. Kinda reminds me of the recording industry's response to Napster.