Because, believe me, it's pretty easy to find out who someone is based solely on the username they use. Not only that, but they'll usually leave enough information in comments or profile pages that - while seemingly useless on its own - can help pinpoint who they are, where they live, and what they do.

If the internet has shown me only one thing, it's that it doesn't matter how insignificant you think you are; give someone the opportunity, and they will find out everything about you...just for the fuck of it.

I've been on the receiving end of this myself, simply because I said something that pissed off a /b/tard-wannabe. Fortunately, all the info they were able to find was my first name and email address, so the harassment was easily contained. But I won't lie when I say it was rather disconcerting to have complete strangers trying to dig up information on me.