Just curious, has that aspect of the GPL ever successfully been held up in court?
http://en.wikipedia.org/wiki/Copyright_law_of_the_United_Sta...
Copyright grants 5 exclusive rights to the authors of a work. Two of them are creating copies and distributing copies. Two of the others don't really apply to executing software (public display and performance of audiovisual works), and the last is the creation of derivative works.
What the owners of some copies of code do with them is not something the author gains any control of. That can only be restricted by the license, which as it says, is governed by applicable contract law.
Also, per se is an important qualifier in that sentence: http://en.wikipedia.org/wiki/List_of_Latin_phrases_%28P%29#p...
In the case of software licenses such as the GPL, it's a weird amalgalm of the two. The only thing which gives you the right to make a copy of the software and use it is the license. If the license becomes invalid for whatever reason (eg. revoked by Zed) then you have an illegal copy of the software, and are pursued under copyright law, not contract law.
Your original point (http://news.ycombinator.com/item?id=4242918) is certainly wrong - Zed can revoke the licence at any time, and you have to stop using the software.