I've never heard that Microsoft pays OEMs but they certainly do give them a discounted price. Also it is my understanding that they do have some contract terms that either forbid sale of the same hardware with other OSes preinstalled or forbid sale at a lower price with other OS or no OS.

This is all from years ago, I don't know what terms are these days. Obviously Dell, HP, etc. do have a few preinstalled Linux systems in their catalogs now.