If Apple were to charge for the update to OS X after Microsoft — a company notorious for high software prices — made its own update free, Apple would appear quite miserly.
I like the notion that Apple followed Microsoft’s lead here — and was actually forced to — as if Microsoft had any choice other than to try to correct the shitshow that has been Windows 8 with a free update. (Much like Apple did way back in the day with OS X 10.1, by the way.)
You could also argue that these Windows 8.X releases are more akin to OS X 10.9.X releases, which have always been free.
But the key point is that Apple has now stated that all versions of OS X are going to be free going forward. Do you think Microsoft is going to do that with all future versions of Windows? Considering that selling that software is one of their core businesses, it’s hard to see how they could possibly do that. Which is why they need the hardware business to work.