It's also a very common word coming from the romance languages, with English being the strange one that doesn't make a distinction between free as in beer or as in speech. And in spite of what you may think, free as in beer on Windows means freeware and that kind of software has really bad connotations, hence the distinction is kind of necessary even from a branding perspective.
And btw, Occam's Razor applies: if LibreOffice has less success than OpenOffice on Windows, that's probably because there's no Sun anymore to promote it.