check the laws in all 50 states and get back to me on that would ya...
Most states have some law requiring vehicles to keep right unless passing. They vary from state to state.
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 ... "
Note that it says "less than the normal speed of traffic." It says nothing about whether the normal speed of traffic is slower or faster than the posted speed limit.
All of the states in the US are members of the National Committee on Uniform Traffic Laws and Ordinances who publish the Uniform Vehicle Code, though not all of the states have implemented the Code completely.
For instance, Florida 316.081(2) 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 or as close as practicable to the right-hand curb or edge of the roadway except when overtaking and passing another vehicle proceeding in the same direction or when preparing for a left turn at an intersection or into a private road or driveway.
So, if you're tooling along in the left lane of the highway with your cruise control on and another vehicle comes up behind you, you have to get over into the right hand lane.
Failure to do so is a violation of Florida law.
If you are in the left lane and not passing, you are breaking the law.