A couple of hypotheses, off the top of my head.

1. NDAs - if someone's in a closed beta, and starts posting screenshots, they can quickly identify the culprit.

2. Hacks - if someone anonymously boasts about finding some exploit in the game, and shows screenshots, they can be tracked down.

3. Abuse prevention - if someone posts screenshots of themselves abusing another player, or breaking the TOS in some other way - but with names blurred out - it would still be possible to find out who it was.