Re: "no requirement of 'registration' for deactivated firearms"
while I agree with the sentiment, the fact is, it is possible to re-activate at least some of those guns. One way to do it (and it's probably done) is dismantling such a piece and shipping it abroad, where different rules apply, and parts which have been welded / made dysfunctional, can be removed and replaced with functional ones. This is probably easier for guns bought several years ago, when rules of how to disable them were less stringent.
p.s. and I happen to be a somewhat interested party, because I have such a device, which is, legally, not even mine (I bought it for a collector friend long time ago, and he paid for it at that time). But, as he happens to live abroad and with other matters at hand, he's not figured out how to take it out of my hands, so to speak, especially now, as the rules changed recently and, apparently, you can't sell or bequest deactivated firearms which were bought more than a couple of years ago, or something.