“In fact it can reinforce their opinion . . . There are people who actively reinforce their own views by reading the opposite side of the argument.”
This goes back to the classic bullying concept. Knowing that you are upsetting someone else makes some people almost feel better about themselves. They know they can set someone off just by leaving a single comment, even just 1 word at some times. They seem to feel more important because others disagree with them. It's not necessarily about being right, but about knowing its bothering other people and getting a reaction out of them.