If someone sells you something, and you decide to modify it, destroy it, or copy it for your own use, then it seems like that is the owner's prerogative. If you want to copy it for resale or trade purposes, then that's a no-no. If you want to copy it to give out as gifts, well. . .that seems like a gray area, but I personally wouldn't care for it if I were the artist who originally created the thing. Because, putting this case with the 8 year olds aside, who knows who folks would be giving these things to or what those other people would be doing with it.
As for asking the artist whether they would approve of a guy recasting them, I think the context does matter. If you're just doing it for yourself to experiment with different mods or to put different figures together--particularly when you're talking about a rare sculpt that you can't get extras of--then I don't see why the artists would get so upset. What's the next logical complaint they might raise? That they can't be painted or repainted, because it would sully the integrity of the original product? That it can't be glued to a neck post for the same reason? That they can't be altered in any way, even by accident (say, scratched or scuffed), or the owner should return it to the artist? If you own it, and if you aren't giving it to someone else, then you can do what you want with it IMO.
As for asking the artist whether they would approve of a guy recasting them, I think the context does matter. If you're just doing it for yourself to experiment with different mods or to put different figures together--particularly when you're talking about a rare sculpt that you can't get extras of--then I don't see why the artists would get so upset. What's the next logical complaint they might raise? That they can't be painted or repainted, because it would sully the integrity of the original product? That it can't be glued to a neck post for the same reason? That they can't be altered in any way, even by accident (say, scratched or scuffed), or the owner should return it to the artist? If you own it, and if you aren't giving it to someone else, then you can do what you want with it IMO.