Without also requiring bikes to come with speedometers* I think this is absurd. The whole reason speed limits for motor vehicles is a reasonable concept (regardless of the speed limit itself being reasonable) is that all motor vehicles made since about 1910 have had them a speedometer. Of course you can tell if you're moving 30 MPH versus 15 MPH, but speeds marginally faster than 15 MPH just don't feel significantly faster. Without the ability to see your speed, and more importantly, the expectation that everyone on the trail can know their own speed accurately, this smells of an easy cash grab by law enforcement. I'm not even saying I think speeding on a bike on a mountain trail is safe or a good idea. I just think it's unfair to enforce something like this if your average person isn't going to be able to make sure they're complying like your average driver of a car can. If no one mountain bikes without a speedometer then I suppose my point is moot, but I'm not a mountain biker so I wouldn't know.

*A quick search on Amazon reveals many for less than $20, but I've never seen a bike sold with one attached.