The GPL was very carefully written so as to prevent anyone imposing restriction on what anyone else can use the software for.
The whole point is that a commercial licence is all about restricting the user - they read as "you can't do X, you can't do Y, ..." and this usually goes on for page after page. If you look back through history, pretty well all "new stuff" is built on a foundation of "old stuff" - the old "Standing on the shoulders of giants" saying (
http://en.wikipedia.org/wiki/Standing_on_the_shoulders_of_giants).
Commercial licences are all about keeping everyone else with their feet on the ground. The GPL is about making it expressly allowed to stand on the shoulders of those who've put work in before. If you were to try and impose a "we take a cut of any commercial receipts" clause then that would have two very important effects.
The first is that it would undermine the principal bit of the GPL - that you may not restrict how and for what someone else may use the software.
The second is that it would open the biggest can of worms imaginable. What is "for profit" to start with ? I'll give a real example of something I'm working on right now ...
I'm setting up a small box that we can plug inline with a customer's internet connection and it will log their traffic - broken down by IP address, and eventually by service. It's running Debian, and the customer will be fully aware of that - and if they want access to the box then they will get it (subject to the usual "don't pork around and break it" talk first).
So, what parts of that are for profit ? We won't be charging them a penny for the software - but we will be charging them for the hardware and (some of) my time to set it up.
Secondly, if it were to fall within such a "for profit" rule, then who would get the money ? Will it runs the Linux kernel, so that's one chunk. It relies on coretools, so that's another. Then it runs Shorewall, so should Tom Eastep get a share as well ? And RRD Tools, so a share to Tobias Oetiker ? There may well be other "primary" packages involved, and then a load of dependencies as well - so where do you stop ? Do I end up paying 1c each to many different developers ? How will that work given that it would cost orders of magnitude more than that to simply send them the money ?
Or should the money go to some central collecting agency like music royalties ? Same issues will apply in determining who should get the money - only this time there'll be the middleman to take his cut first. I think we all know how well that works in the music industry.
