The road into Black Hawk from Denver is a long uphill, so bikes moving in that direction would be going slowly. Methinks the real motive here from Black Hawk is to cut down on the number of bikes on the highway, period. Personally, I think the cyclists are nuts, but they are engaged in a lawful activity according to the State of Colorado, so Black Hawk is akin to the City of Philly trying to pass its own laws in defiance of Pennsylvania gun laws.
But the road back is then a good downhill where bikes can easily reach 50-60 mph.