Who cares about being an ass in forums?
Remember a few weeks back when some kid was arrested for plotting the death of another kid based on "killing his avatar in CounterStrike?"
Now imagine playing Diablo 3 or StarCraft 2. You beat a guy badly in SC2, or maybe in Diablo 3 someone PKer kills half your party then you slaughter him and take his items. Nothing really out of the ordinary. But this is battle.net, and you're put into a game with random people, and it turns out to be someone completely deranged. He gets your name. Maybe he doesn't do anything crazy, but he finds some way to harass you. He orders dog poop shipped to your house, or he calls at 4am every day for a week. Still a huge hassle. And all you did was play a game better than he did.
This isn't comparable to Facebook, where you only interact with those you already know and choose to. This is a video game matchmaking service.
Whatever. Right now it's optional. I can't imagine legal forcing it. They should be terrified (100% this is the devs idea.) And there's no way they can stop you from using fake names, so I'll just do that with Diablo 3. I'll register as John Birshire...