Violating a contract is not "illegal", but is a civil offense, and requires the other party to sue you to recover damages. Copyright infringement is also not "illegal", but is a civil offense (however, i do believe that a lot of copyright lobbies want to change copyright violations to be a criminal offence, so they won't need to sue, but instead use public prosecution to recover their damages).
> The unauthorized reproduction or distribution of a copyrighted work is illegal. Criminal copyright infringement, including infringement without monetary gain, is investigated by the FBI and is punishable by fines and federal imprisonment. [0]
[0]: https://www.fbi.gov/investigate/white-collar-crime/piracy-ip...
(iii) to install, use and run up to two (2) additional copies or instances of the Apple Software within virtual operating system environments on each Mac Computer you own or control that is already running the Apple Software, for purposes of: (a) software development; (b) testing during software development; (c) using macOS Server; or (d) personal, non-commercial use.
Also, following these instructions doesn’t necessarily violate Apple’s license. You can run Linux on Apple hardware, and install Mac OS X on top of it, can’t you?
Finally, IANAL, but software licenses likely aren’t enforceable to consumers in large parts of the world (a shrink-wrap “take it or leave it” probably wouldn’t be considered an agreement between parties in many courts, and that’s what you need to call something a license)