Context: This discussion originated on the emacs-devel mailing list, where there was a discussion of adding code to company-mode (text completion) explicitly supporting Clang [1]. It was pointed out [2] that RMS has in the past shown a disinclination to incorporate Clang support into GNU projects because of his perception of its encouragement of proprietary software riding the coattails of open source (but not Free)…
I think the biggest hurdle to people caring about copyleft is the sheer tonnage of disrespect people like RMS show to others. Nobody wants to work with a curmudgeon.