Ed. note - a brief response to Tim. A) my name isn't Timothy. (I know, I know, we all look alike. :) And B) I was trying to say pretty much what O'Reilly is saying - that all licensing, including the GPL, is an expression of power over what other people can do with the software. Hence the term "all licensing". If there were no copyright whatsoever on computer code, no intellectual property considerations at all, perhaps we could approach the state of true freedom. In the meantime, the GPL is a good way to place code firmly into a state where it is mostly free - you are free to do anything with GPL code except take it out of its free state. As far as restrictions go, this one is infinitely more palatable than most of the powers that software licensing seeks to exercise over software users.
As a more general point, I take issue with O'Reilly's description of copyright law as a compromise between creators and users. There's absolutely no evidence that the rights of users are considered when copyright laws are made. All copyright law changes made in my lifetime, nearly all copyright law changes ever, have been expansions of copyright law - if it's a compromise, it's an extraordinarily one-sided one. (I suppose you could a describe a mugging as a compromise between the mugger and the little old lady over rights to her purse.) Copyright law is more accurately described as a compromise between copyright holders and copyright holders. Other descriptions are both inaccurate and do a disservice to efforts to reform the laws.