You hit it right on the head. We don't need no steeking laws to cover our OC. There is no law against it (sans Denver), so it is purely legal. CC instructors get paid only if people come to get their classes that are required by law to CC. They like the CC requirement because it puts $$ in their pocket. Granted, they also conduct other non-required classes, but since we're talking CC classes only, we'll stick to that.
Many folks would like to see a law protecting OC similar to what preemption does for CC, but since both the US and CO Constitutions protect the RKBA, a law would be another form of permission and therefore infringement on a low level. The only upside to OC preemption would be more leverage legally against the few LEO's, judges, and legislators that would abuse our natural rights.