Microsoft's business model is selling a licensed OS, something which honestly is getting harder every passing day. Apple's business model is selling compelling hardware at a premium price with their own software preloaded.

Apple could (and now indeed does) give away their software for free and still make a decent living. Their software wont run on any hardware they haven't already made a profit on, so there's no significant "piracy" risk either way.

That said, Windows 8.1 was a free upgrade for everyone already on the Windows 8 bandwagon (and thus in need for some overhauls).