to use your argument, if they did create a combined album (with properly licensed songs on it) and then you attempted to tweak it (as opposed to creating your own compiliation from scratch), they could prevent you from doing so.
the flaw in their argument is the 'no additional restrictions' clause of the GPL. they can have that compilation copyright, but if they put more restrictions on it than are on the GPL, they then loose the ability to distribute the GPL work (unless they have some other non-GPL license to the work, which does not exist for the kernel)
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