Why do you think it should be free?
Because macOS is? And that's rock solid/never crashes and just works? I have literally left machines on for over a year at a time before without even putting them in sleep, and never had a single issue across multiple machines. Even though I love linux, I've never witnessed that kind of stability/continuity.
Even when I build hackintoshes, once they're setup, they just work, and always have done, I started off with hackintoshes before buying a real mac, even installing 'unsupported' never versions of macos on ancient hardware, still doesn't cause an issue even with the thirdy party scene tools required to do it!
My mid 2008 unibody is still kicking to this day and will happily do a hell of a lot considering it's age. Can't find many windows machines that will do what this will. And I can still put windows/linux on it if I please.
You can legally use 1 'copy' on as many machines as you want. There's not even a serial number.
Even years ago when it was paid for, it cost between £12-36 and just continued to become cheaper and then became free, even then, there was never a serial required... You could legally back in the day even use 1 'copy' for 5 people in your household. Meanwhile MS were releasing what 7 or more versions of vista/7 all with ridiculous pricing?
So yeah, why should be pay when you don't for a clearly more stable/preferred os on another platform? The sheer effort put into linux as well versus being made to sell for profit, and yet even that is still free. So yeah TLDR MS have no excuse, it's just greed and bloatware.
I will only ever use a oem/retail key off a site ever for any Windows vs paying the rip off they want. M$ fall into the same category as the BBC for me...