I don't know if that is entirely correct. I believe you are supposed to keep right except to pass, but I'm sure it differs from state to state. I would also bet that passing on the right is illegal as well.
I was driving home from working on cars late at the shop one Friday night/Saturday morning. It was maybe 3AM. There was NOBODY else on the road. I was driving in the left lane because the right lane was full of potholes. I had a cop pull me over for driving in the left lane when the right lane was available. Now, we all know he just needed some semi-legitimate reason to pull be over because he thought he had an easy DUI. If only they would enforce that all the time. The worst is the two lane (in each direction) road in the town I live in. You'll get a car in the slow lane going 55mph and then somebody doing 55.1mph in the fast lane that takes them three miles to overtake and then they still camp in the fast lane.