Is that the law, or just how it's turned out?
At first I thought it was passive-aggressive behavior, big SUVs and the bigger pickups would slow left lane driving to less than the limit and line up with other slow drivers to create rolling road blocks. Eventually, most vehicles drifted to the left lanes to the point where the right lane can be empty and faster traffic can please itself.
I’m usually driving the limit or more and don’t particularly care for this setup, with the entrances now being in the fast lane, but it’s just the way it is.