[CuriousCosmic]

But the thing to note is that a user can have a right to distribute (as with GPL) but does not necessarily have the rights to the license.

So if the user uploads the source to GitHub, they agree to the terms (which they may not actually have the rights to) but that isn't equivalent to the rights owner giving GitHub the rights to distribute the source under a different license.

The TOS can only modify those distribution terms (if it even can be found to be legally binding) if the user uploading the source is the rights owner which in so many cases is not the case.