It's a case of the microdevil they know versus the Linux they don't. If they have Windows I don't have to explain anything to them about how to use it. Cuts down a LOT on after-transfer tech support. And it's free to them so I don't get paid for tech support either.
Actually I never bother even erasing Windows from the internal hard drive. I just run Linux Mint (Cinnamon) from a low-profile flash drive.
And about LMDE specifically, that would spike the tech support calls even more. It never did really get the rough edges smoothed out. It's only a test bed to answer "what if Ubuntu vanished and we had to work with a straight Debian base?" If I gave them a computer with Linux, it would be mainstream Mint with a Cinnamon desktop environment.