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.
It's not perfectly legal, but reasonably more legal than the alternative (a key bought from a reseller). Those stickers are meant to be affixed to a machine sold by a reseller, and is only meant to be used for that machine. If the machine is destroyed and the sticker/key is kept, then it can be used, and it does register Windows Professional/Home installation rather than an Enterprise/Education installation, but is in violation of the TOS that it's provided under.
That being said it is difficult to shed a tear for Microsoft of all companies over this practice.
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.