There is one circumstance that reveals how half-baked this law is. When I've passed someone in wet or dirty conditions, and if there is no one catching up to me, I remain longer in the left lane so as not to spray water, crud, rocks etc. on the car I've passed, before I cross back into the right lane. This law makes such road courtesy illegal.
(In addition, on dry roads with patchy wet spots, I make some effort to avoid the wet spots so as to not spray water on cars behind me.)
I'm also wondering if, where this law already exists, anyone driving in the left lane at or above the speed limit has been ticketed for being there and not in the act of passing. If this law really was to keep slowpokes from obstructing traffic in the left lane, it would say something like: "anyone traveling at or below the speed limit in the left lane, and where it is possible to move to the right lane, shall not obstruct overtaking traffic by remaining in the left lane."
Instead, it says you can't be in the left lane unless you're passing. The difference is that in the real world, this requires law-abiding drivers to facilitate speeders. Isn't this the same as expecting people to hold the bank door open for escaping bank robbers? Seems to me anyone ticketed for left lane hogging, who was at or above the speed limit, could challenge the ticket on this basis.