RMS: Companies that develop free software and release it under the GNU GPL sometimes distribute some copies of the code in other ways. If they distribute the exact same code under a different license to certain users that pay for this, typically permitting including the code in proprietary programs, we call it "selling exceptions".

Copyright assignment can be ethical, and can unify a project under common ownership, or it can be misused to impose control and bypass the GPL, indemnify the code against patent infringement, and subvert the developers' intent in contributing to an 'open source' project.

A little while back, Michael Meeks published a lengthy piece about copyright assignment (not nearly as lengthy as the articles he links to on untangling Wittgenstein's net). Go on, read it (Michael's stuff, not the net). It's worth your time. When you get to the bottom, follow the link to Dave Neary's take on assignment as well.