I would call this similar to (but not as bad as) when the US started scooping people up, imprisoning and torturing excuse me, enhanced interrogating them, without trial or any due process. The Bill of Rights lays out what the Government is not allowed to do. The Government claiming they are allowed to do something they are specifically proscribed from doing because they are doing it outside the US or to a particular, unloved, group of people is quibbling, and reminds me of the kind of argument a child caught misbehaving makes.
We routinely pontificate about how there is a natural right to own firearms, and the government can not infringe on it. I have read the argument on this site that folks in Europe and other restrictive countries have the right to bear arms, their government's are just using force to infringe on that pre-existing, natural right. If that's the case, I can't agree that there's a blanket prohibition on those folks exercising their right without due process.
If the courts were to make it a condition of the illegal's release after a hearing, pending the outcome of their full case, I could at least see that as lip service to due process.