I think it's a bit of both ignorance and stupidity.

What bugs me is when I'm in my car approaching an intersection and there's a cyclist to the right of me, I'll slow down or even stop before I try to initiate a right turn so the cyclist can go straight without me cutting him/her off. This sometimes holds up the traffic behind me and I've gotten honked at. Like I'm supposed to take out the cyclist so the car behind me can go?!