For $25 I can get a legitimate Windows 7 Pro license sticker that works like any Windows 10 license, which is perfectly legal all the way through.

But depending on your jurisdiction, the $5 enterprise keys can also be entirely fine. Sure, in a way that's outsourcing piracy. But the law doesn't have to see it that way. There's nothing illegal about buying enterprise keys, and if they are overprovisioning keys that's between them and MS, I can't even know if that's the case.