Perhaps the type of people who wear confrontational t-shirts do other things that result in confrontation. If I'm going to wear a shirt that's a big middle finger to others who are not like me, then I probably act similarly towards them in other areas of my life. Even if I think I am treating them the same. If one group I despise is car drivers or SUV drivers, perhaps that would extend to how I cycle when around them, whether I realize it or not.

I remember that study that came out a few years ago that said people who drive cars with bumper stickers are more aggressive than other drivers, even the peace and love type stickers. Not sure how great a study it was.