" But what if I gave YOU butter for free, but under a license (i.e. the GPL) which improves your popcorn. And you in turn gave it away for free along with the popcorn you produced. (which is allowed).
Then you decide to start charging 10 cents for the popcorn, and are still including my butter. That's not ok. It violates my license."
The gpl does allow for commercial use, it just doesn't allow the product using the code to be proprietary. So selling the popcorn would not be in violation of the license.(well, assuming it covered popcorn :P)