copyright laws
This was a thought that also crossed my mind. As far as I can remember, it's a simple principle (in the EU or NL): The creator of an artistic work is also the owner. Which means at third party is not allowed to copy, sell or claim this work to be his/hers/its. When artists sign a label contract, they usually sign a contract to sell their music to the label for a certain prize (usually an x amount per sold copy). The ownership rights of the music is than transferred to that label. Which brings me to Embers quote:
Faulty premise unfortunately; if Metallica wanted to allow us to do that then they would have to agree to the terms of the license. Basically Microsoft says if you want to mix things with their IP, then you relinquish your rights to it. Also like I said, you don't explicitly transfer ownership, you just relinquish all rights that ownership entails.
If I am right the terms of license basically have the same effect as signing a label contract, with the exception of actually transferring ownership. You just agree not to complain when MS uses your assets and won't pay you or give you credit.
I am not sure how the UK law works when it comes to contracts and agreements, but since they're in the EU I think it's safe to assume that their laws are similar to NL laws. In NL, whenever you agree to sign a contract, everything you agree to in that contract is legal (if it is not alien to international, European and other national laws).