[VanL]

Quick summary:

1) It is problematic to use copyright infringement as a hammer to force people to release/relicense code that is not related to the copyrighted code. (That's the "misuse" bit)

2) If you try to do this via contract, there are lots of practical difficulties associated with actually releasing the code - the biggest of which is that you probably don't own all the code you would be required to release. (That's the "impracticability" bit)