It is a law everywhere that you must pass on the left. Slower traffic keep right. It is a cultural issue.
Also, I pass people on the right all the time, but I blame them for it. :-) If people don't want to be passed on the right, they shouldn't drive on the left. Simple enough.
Laws for passing on the left was intended for roads with one lane going in opposite directions.
Driving the speed limit is considered normal speed according to case law. Slow traffic is going 10 on a 25.