It astonishes me that people so often get confused between two entirely different concepts:
Legal is what the law says. Law is made by the properly elected representatives of the people within the confines of the constitution.
Endorsed by an EULA is what a particular individual citizen or corporate body happens to write down on some paper. It may or may not be legal, it may or may not be enforcable under the law, it may or may not have any relevance at all to that which is "legal" or "illegal".
It is, in fact, perfectly legal to do absolutely anything you want with a copy of Windows, saving only those things that are specifically prohibited by law. You cannot, for example, use it to bludgeon your accountant to death (because that is prohibited by the law against murder); you cannot throw it out your car window (because that is prohibited by the law against littering); with certain exceptions for fair useage, you cannot reverse-engineer or copy it (because that is restricted by the copyright laws); and you cannot use it to strangle your dog (because that would be to break the laws on animal cruelty).
Your are perfectly entitled to sell OEM software without bothering to incluse even so much as a screw with it. There is absolutely nothing illegal about that. However, Micro$oft are then perfectly entitled to refuse to sell you any more copies of it, and to hound you with frivolous nonsense suits which will soon mount up into something which you are unable to defend yourself against (because they can always afford more and better lawyers than you can) and which make it impossible for you to earn your living.