Typically, you stay out of the left lane unless passing in the US. It's that easy. In many states, the person failing to yeild to faster traffic is ticketed, but not all.
Most of them go back to the right/slow lane when they're done passing any trucks and other slow traffic. Even on the I-5 it's not like the right/slow lane is an unbroken line of trucks from Mexico to Canada, you know.
Then why the f*ck are they in the left lane?!