I can see merrit in both sides of the coin. But I'd hate to rule that sponsored magic-users are free from the Laws, by default of not having their own power. Sponsored magic already confers many advantages, and whilst it has disadvantages I don't think they should be free of the Soul tarnishing affect. Although they're likely out of the White Council's jurisdiction goes, as far as punishing them goes. Non-mortal (or semi-mortals) are of course, also free from the stunt.
I guess it depends on one fundamental question: can a sponsored magic-user cast a spell they do not believe in;
a) of their own free will?
b) when it is demanded/needed of them, by their sponsor?
The answers to the above, would highly mold my opinion. Unfortunately we have not (to my knowledge) discovered the answers in the books. So I guess it is up to your table to decide.