Basically, if you want to do something that copyright forbids then you can't, unless you get permission. The GPL (and other share-alikes) gives you permission, as long as you obey its rules. Break the rules and you no longer have permission, just as if you were paying a monthly fee for permission and then stopped paying.
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"The GPL Is a License, not a Contract" by Pamela Jones
http://lwn.net/Articles/61292/
Basically, if you want to do something that copyright forbids then you can't, unless you get permission. The GPL (and other share-alikes) gives you permission, as long as you obey its rules. Break the rules and you no longer have permission, just as if you were paying a monthly fee for permission and then stopped paying.