If it were open source it would be legal to copy it. Large corporations and OEMs that wouldn't consider pirating Windows would be happy to switch to using it for free.
Surely that depends on the license though? It would be possible to license open-source windows in such a way that corporations and OEMs couldn't distribute it without paying a license fee.
Such a license would not be an open-source license.
What? Surely this falls under the free-as-in-beer vs free-as-in-speech distinction? What about RHEL? -- that operates on a paid open-source model.
And there's multiple free version of RH that are basically identical other than the branding and support.