Because the people posting behind "Anonymous" are insecure. When I first joined this site, before I switched accounts, I posted asking whether or not I should excersize because of a bit of pudge in my stomach area. All my life I have been "skinny", and i noticed my belly was poking out, I may have seen it bigger than it actually was, but anyway this bitch decided to go anon and say that I was fishing for attention and compliments when I wanted some honest advice. Obviously you can tell that she is not comfortable with herself and is jealous of other people's bodies. If the person who said this is reading this Fuck you... I'm sorry to the asker I had to vent.
The two times I've felt most mistreated on here were actually by users who weren't anonymous. But yes, some anons can be mean, too. :p I assume it's because they don't have to "answer" for what they say.
I tried my best to find a screenshot from V for Vendetta at the end of the comedy sketch, peeking out under the theater curtain with a cigar and snickering (the best representation I can think of for the thought process behind what you're describing), but this was the best I could do. www.adancersprism.com/.../anonymous-mask.jpg