Win10 is available free, and legally.
Without a license, you can't activate it. That restricts some functions, like personalised desktop settings, although you even have a 30 day grace period to do that.
It also effects some other Microsoft services. I'd be fairly confident running cnc software would be fine (no doubt the fine print mentions personal use only)
A response I think to the internet of things, home automation, the rise of single board computers with Linux and Android.
-russ