Why would anyone give SSN to AT&T? Do they also process your taxes? If not, they have no place asking or retaining this information.
When I first got my iPhone, the Apple Store reps could not figure out how (or wouldn't admit to knowing how) to sell an AT&T contract without a social security number. They sent me down the way to the AT&T store who also couldn't figure it out without calling in to a customer service line and escalating to a supervisor. It took over two hours to buy the damn phone without a SS#, but would have been five minutes if I had given it up. Eventually, they admitted that they have a placeholder number they can use instead of the SS# and we completed the transaction.
Granted, this was a few years ago, but I don't see why they'd be any more cooperative today.
So that's why people give it to them. Is it required? No. Do people have several hours to waste and the stubbornness to jump through the hoops? Not usually.