Indeed Free/Open Source licences are not public domain, and saying something is free isn't a complete description of a licence (this goes for any freeware or other material that is offered "for free", it doesn't imply that you can do what you like with it). The GPL is about preserving freedom for end users of software, which isn't the same as freedom in a licence for redistribution, that you get more in licenses like BSD. It's impossible to have both, so one has to choose
But it's good to know when you choose what license to use, and its far from just "it's free".
I agree that mholme's description of the GPL was just plain completely wrong.