Wow, the amount of ignorance displayed here is significant. It's painful just listening to it, and the blatant double think -- I'm protecting my rights by disregarding your rights...
If calling a function from a GPL project or having css/html that interact with a GPL project means that your project needs to also be open sourced, this should be a wakeup call to all businesses to not get anywhere near the license.
If the portions he used only contain function calls, I wouldn't consider it derivative work.
(I linked this in my other reply to you as well. Sorry for the repetition, but I think it was worth mentioning in both places.)
Well, the RIAA considers copyright infringement "theft". It doesn't make either of these true.
The murky legal problem with the GPL and other free software licenses is that there isn't any consideration (no exchange of money), or proof of offer and acceptance.
It's a lot easier to prove with paid software and the clicking of a checkbox.
IANAL, but it would be interesting to know if there's any legal precedents for the enforceability of free/gpl/open source licenses.
The GPL is well-tested in that respect.
My presumption on some of the cases where the GPL has been most successful without going to judgement is that those have been the ones where there is the clearest infringement (i.e., Linksys 'borrowing code' for their routers, etc), i.e., direct copies or only slightly modified versions of the original source.
If it gets far enough, I think it will come down to a layperson's (judge or jury) interpretation (based on lawyer and witness arguments) on whether a theme is like reselling a modified car (i.e., Ruf Turbo - a modified Porsche), or an aftermarket car stereo (i.e., one that reverse engineered an automaker's proprietary connector). Yes, code is intangible, unlike a car, and there are places where my overly simplistic analogy breaks apart, but it's not unheard of to look at it in this manner. A federal judge recently compared mp3 torrent sharing to playing music in a business - not that similar, but in the grand scheme of things, you can see some parallels.