I'll ask the FFTW guys what do they do about external contributions.
Also, thinking about it more, there is nothing wrong with copyright holders having extra rights as long as they do not use these rights abusively. After all, they did the work, so they deserve to be compensated for it. Nobody would dispute that. It is problematic if they use these extra rights to try to make people pay for proprietary software that is not otherwise available freely. This is what I dislike about what Oracle does.