Free OS updates have been the rule at Apple for the last couple of updates, since Lion, I believe. From 2013:

Prices of Apple’s Mac OS X have long been on the wane. After four releases that cost $129, Apple dropped the operating system’s upgrade price to $29 with 2009’s OS X 10.6 Snow Leopard, and then to $19 with last year’s OS X 10.8 Mountain Lion.

Microsoft — the king of the operating system in the ’80s and ’90s and on into the aughts — still charges PC makers who sell the Windows OS preloaded on their desktop and laptop machines, but that business is shrinking, thanks in large part to the continued success of Apple. And just last week, Microsoft announced that, much like Apple, it would not charge consumers who upgrade their machines to the latest version of Windows, version 8.1.