The Uniform Vehicle Code states:

"Upon all roadways any vehicle proceeding at less than the normal speed of traffic at the time and place and under the conditions then existing shall be driven in the right-hand lane then available for traffic ..."

This is the law in most states (maybe not all). On top of that, it's basic common sense. Slower traffic keep right - not rocket science. Left lane abuse is a plague on our rapidly expanding society. Preventing it makes traffic flow more smoothly and safely.