I think GP meant that registration will make people much less willing to get into the hobby, which will dry out the revenue for quadcopter-making companies, making them raise prices or close shop altogether. The few people who really want to play with those machines will prefer to order cheaper DIY kits, and since you can't effectively regulate the components they'll be flying whatever they could get assembled instead of tested constructions with proper safety measures. Also almost no one will bother to register DIY drones, since FAA won't fund an army of inspectors to check for license every time someone takes a quadcopter to the park.
That's how I understood the comment.
That won't be enforceable.
I don't see why not. If an unlicensed drone is found operating off of the owner's private property, it's confiscated. Straightforward enforcement.
It's slightly more complicated, seeing as how shooting them down with nets is a bit difficult (although we may see widespread application of serious DEATH RAYS, excuse me, lasers for that (hey, I want the future that was promised to me in the '60s to come to pass)). There's also the ECM approach, or maybe even EMP.
Rather, follow the drone until it lands. If the owner is stupid enough to bring it back to him, you can go quite a bit further.
How would you find the drone owner and/or operator, exactly? I can see this being used to further erode the right to not be stopped and frisked without probable cause. Now, a drone flying overhead in a public place is probable cause to stop and search everybody in sight.