GFDL 1.3: Wikipedia's exit permit

In case of OpenStreetmap this would not work. People would never assign their copyright over to a foundation (apart from the legal difficulties in some parts of the world to do this).

Some do not want to run risk of having their contributed data under a non-sharealike license, they would rather quit and pull our their contributed data. These would never sign over their copyright. Plus it makes the barrier of entry to the project much higher (see the FSF where you have to send faxes to the foundation, granting them copyright etc...)