In all real leagues, around the world, there are transfer limitations, much stricter and more "unfair" than the one i proposed.
Without even getting in the "fair play" subject, that is the main concern here and any breach of it has become part of written rules and regulations of the game.
In addition, i don't see any constructive input. I can't even tell whether you recognise the existence of the problem or not. Your only opinion on the subject, that i know of, is that we shouldn't have NT-U21 teams and tournaments at all.
Fair enough, but i also believe its redundant to aplly any critisism to any proposition, on this subject, since you trully believe it shouldn't exist as a subject, to begin with.
If my criticism of the proposal was "omg, international play is teh sux!" then you'd be correct. But my concern about the suggestion, strange as it may appear, is exactly what I said it was to be - it's saying that because of some rogue actors that are sabotaging the NT/U21, we must limit the ability of all users to purchase NT/U21 players from other countries. This, essentially, is telling users who might legitimately be interested in purchasing those players that while they aren't intending to do anything wrong, their actions nonetheless can't be permitted because the international game is more important than your silly little club.
Of course, if you're more interested in actual constructive input, what about this scenario: the NT manager from some small country sees a nicely-trained player from his country on the TL at a fairly low price. And we're not even talking NT/U21 caliber player, but a nice SF let's say. The NT manager adds the player to the NT, makes the initial low bid, and walks away with a steal. Sure, this may be no more of an abuse of the system than what's going on now, but instead of a NT being hurt, actual club users (both those who train/sell the player and those who would otherwise buy the player) are harmed. And this isn't even preventing NT sabotage - it's just an opportunistic move by an NT manager to enrich their club interests.
As far as the restrictions in the real world, I've got no problems with restrictions that are set up to improve the game - not letting teams in leagues with a 50k TV contract place a bid on a guy with a $200k salary (NT or not!), regardless of nationality, for example is something I'd support. But adding restrictions this widespread to address a game problem that only really matters for the NT/U21 is trying to crack an egg with a hammer.