they cant actually enforce it-its a way of identification if you did something wrong. if you don't register, you wont run into any problems. if you crash and you aren't registered, you lost your drone but you are fine. if you do register, they will identify you and fine you.
also this is a policy, not a law. it is to identify the owner of the drone IF you break a law. not registering is completely legal, but the FCC just doesn't like it.
I am not telling you not to register, just filling you in on the info