There's a big difference in what is being discussed on the different sites. For the regular sites, it is definitely important to stay polite and impartial because you are supposed to be answering questions.
On Meta, there is a lot more discussion concerning the future of the sites and how the community should operate. This is when you start introducing opinions and that is when people will start going toe-to-toe on issues, and often times people will take more offense at this conflict than they would about a disagreement over whether 2+2 really equals 4.
On Meta, opinions matter highly, and it is also an avenue for a lot of us to be more vocal and laid back and not have to feel so "professional". I agree that this is not an excuse to be a blatant asshole, but I consider this site a lot more like the "teacher's lounge" where Mr. Johnson and Mrs. Crabtree can smoke and drink their coffee and not have to be the perfect role models that they have to be in front of the students.