Well, we're talking about California, right?
A. I posted this upthread already.
State "keep right" laws
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.
Each state adds it's own restrictions/adjustments/etc. on it (like not applicable on a 2 lane freeway, or only slow vehicles, etc.). Most states (not ALL of them, but most of them) have some variant of keep right except to pass.
B. In Cali, it's "notwithstanding the prima facie speed limits." So if the person in the left lane is doing 105mph, and I come up on him doing 106mph, legally he still needs to move.
This is not related to the HOV lane the OP was in.