[stonemetal]

The main problem with the public domain is that there is no such thing as "the public domain". Each country has its own copyright law, with its own terms and conditions. What the public domain means in the US is not what the public domain means elsewhere i.e. Germany. In Germany an author has Moral Rights which the author is incapable of giving up and may only transfer as apart of their will. So rather than require all German contributors to be dead and have a will that grants their moral rights to the project, using a permissive license gets the same effect.