By that metric, Windows is also free if you buy a Surface.
>their market share has been gobbled up by Apple in the last decade or so
Today I learned that going from 5% to 7% marketshare is gobbling up a rival that has 90%+ marketshare.
There's a difference between Microsoft's model (using a hardware product to better market a software product) and Apple's model (using a software product to better market a hardware product). Microsoft is a software company first and foremost, while Apple is a hardware company first and foremost.
That's the root cause of the reason why your particular comparison isn't exactly valid: that OS X is not at all designed to work on hardware that wasn't built by Apple (and, indeed, getting OS X to work on non-Apple hardware requires a modified OS X and installer on top of matching the hardware of a Mac almost perfectly).