This will surely alienate everyone who's bought a proper copy - who will now wonder what the point was, if Microsoft are going to reward those who didn't.
A better strategy might be to offer Windows 10 free to everyone in the first 6 months or whatever? That way it doesn't seem like they're singling out pirates for special treatment (and they're offering free upgrades to everyone else anyway so I doubt it would cost them significantly more).
They could easily exclude enterprise editions/use which I expect they are excluding anyway.