The question I've always had is how enforceable is that really? Obviously the whole point of Apple making macOS freely available is to run it on Apple hardware. They don't give it out for free to run on other hardware but can they really do anything about that other than require you to enter a serial number to download an image? If they really cared, they would just do something like hashing the serial number and current date and time against a secret key (maybe inside a read-only portion of the TPM) and only Apple would be able to verify that the hardware is legit. You would need to somehow expose the TPM to the hypervisor to be able to generate hashes for macOS to verify it's license. Clearly this is not a huge problem for Apple because they would already be doing this if it was an issue.