[TuringTest]

  "the GPL is designed to globally maximise the ability to
  use, modify and distribute a piece of software. The only 
  ability the GPL does not provide is the ability to take 
  these abilities from future uses."

I would summarize that as "the GPL is designed to globally maximize knowledge about the software, and the ability to use that knowledge".

The part about forcing published forks to release their source code is a consequence of that purpose: hiding the source would reduce the amount of knowledge, and thus it is forbidden.