I've definitely seen fights in the forum. The thing is they're usually dealt with quickly by the mods and the repeat offenders are banned for 1000 years and forgotten by history.
As for the site, I don't think it's strange at all that there's not a lot of drama. I feel like there's not much personal interaction on the main site. You can follow comics, but you can't "friend" individuals. People post their comics, other people react to those comics. Sometimes there's a bit of a back and forth in the comments, but it's usually about the comic characters, not about any real person. People rarely comment on other people's profiles aside from the whole "thanks for subscribing thing."
Also the comments are kind of contained. Two people might comment on the same page of some comic and maybe disagree on something... but then never comment on the same page of any comic ever again. It's not like... say Deviant Art or Twitter, where people tend to run in the same circles and have a lot of chances to rub each other the wrong way.
Sooo, I don't think it has anything to do with the people being nicer, I think it's more to do with how the site it designed.