Google didn't move to clang/llvm because they hate copyleft. They moved because people who work on compilers prefer to work on clang/llvm, and if you have a compiler group you need to keep them happy.
It's probably also true that the FSF is more of a pain to work with than absolutely necessary — they want copyright assignment, have some abhorrent code style, and still insist on tracking useless diff summaries in ChangeLog files long after the invention of RCS systems.
