They are not doing the speed limit. Many States consider the left lane a passing lane and the speed limit is about 10mph higher while passing (some States don't specify it, but still consider the left lane a passing lane only).
If people are driving the speed limit and passing the Prius (or any other car in the left lane), and cop sees it, they will ticket the driver in the left lane.
"Keep right except to pass" is the most ignored law on the books. I doubt may cops even know that it exists. I remember watching a local news segment year ago when a cop did a ride-along with the newscaster, who drove the speed limit in the left lane to demonstrate speeding and road rage. The cop was commenting on how the people passing on the right and giving his driver dirty looks were behaving dangerously and breaking the law.
I have yet to see a cop ticketing a car doing the Lefty thing. I drive lots of miles per year and I routinely pass people on the right, and have been doing it since I was 16.