It makes people upset due to tribal evolution. Ever notice that most people that actually are willing to discuss politics will personally identify with an ideology? Just like our ancient ancestors defended the tribe at all costs (or you died), today, our tribal brains will defend our political tribe (ideology) regardless of facts. And the more facts that contradict the ideology, the more convinced you will be that the ideology is right.

Its called the Backfire Effect. I have a video about it on my blog. Our tiny, tribal primate brains are simply programmed for it, and few people overcome it.