It would appear that no-one has any idea of how many of these drones/uas/toys are around. Most toy shops have a few dozen assorted radio controlled helicopters of around 2 ozs weight hobbyist model shops perhaps a hundred or so. So let's be conservative and say that there are 20 million of these drones/toys in the wild. I do not see how it is feasible to put together an organization that would need to be the size of the state DMVs to maintain a registry of these toys/UAS/drones. But I can see how this would appear to be extremely attractive to an empire building bureaucrat.
Even if it were done - it will not stop someone flying a small radio controlled aircraft into the path of a manned aircraft. But efficacy is not one of the requirements of a bureaucratic registration system.