Well, it's safe to assume that Apple is not giving this update away for free because of the goodness of their hearts. There are two reasons for the free upgrade:
1. Simply because Microsoft gave away the 8.1 upgrade for free to existing 8.0 customers.
2. To lock-in the customers even deeper into all those fancy Apple cloud services. (Exactly what Microsoft also did with the 'free' 8.1 upgrade.)
When a corporation gives away something "for free", you can bet that there are ulterior motives involved.
There's also the fact that Macs count for less than 15% of Apple's revenue these days. Charging money would mean fewer people update, and it wouldn't bring in that much money anyway. Much better to give it away for free, so more people download it, and take the 30% cut of every app sold for the new OS.
Alex