Yes? If you're going to a place where freedom of expression is a primary value, then you should not be surprised by any kind of expression, from cheerful agreement to hateful bile. I agree being rude is well... rude, but people should be free to choose for themselves what rudeness (and directed at whom) they approve of, don't you think?
As to why people would want to participate, I suspect it's for the usual reasons: to accomplish a goal, or to gain influence and social status.
Edit: Surely you agree that sometimes rudeness is warranted, and that people in fact are rude on Twitter?