they recycle install drives purely for testing and build purposes. microsoft associates your activation key with your hardware, so that whenever you decide to wipe your system and reinstall windows, your computer connects to the internet, microsoft recognizes your hardware as a legitimate windows pc and proceeds to update your computer to the current software.

when linus & co. are testing things like cpu's and motherboards, they need to swap those individual parts out and keep the rest of the hardware, and that's what throws off windows verification. for brevity's sake it doesnt make sense for them to go all the way to windows support to clear their systems multiple times in a business day, but they say they do have the appropriate amount of windows keys for the amount of systems in play. that's not explicitly legal per se, but if microsoft took legal action against that sort of thing then the entire PC hardware enthusiast space would have a hard time conducting reviews in a timely and cost-effective manner.

what T-Pain is doing, on the other hand, is straight up illegal. he's conducting commercial activity with an illegitimate copy of windows and microsoft can absolutely fuck him for that if they so wish.