Idahoans have the tendency to block all lanes of traffic because they all think every other vehicle on the road is driven by a dirty Californian. Driving slow by itself isn't rude. But when you drive slow, then speed up to prevent people from passing you, that is rude. And it leads to tailgating, which is also considered rude. Other states would have slower drivers pulling off the road to let traffic go past, not so here. Here there are no laws enforced keeping slower traffic right. And the states propaganda machine is aimed at the mythical "aggressive driver," which is a driver who knows where the hell he is going and has a desire to get there before he dies, not at the douchebags causing the irritation in the first place.
I'd agree that Idaho seems to be overrepresented in the slow assed, inconsiderate driver category.