It doesn't really violate any regulations, because the use of guard is guidance, not regulation, and even if someone's talking on guard below 10,000 ft, it's a broadcast, not a conversation, so it would't violate sterile cockpit policies.
That being said, the FAA can basically do whatever it wants, because they have the authority to prohibit any pilot or airline from operating an airplane, whenever they want, for any reason, because of a rule they wrote that "no person may operate an aircraft in a careless or reckless manner". There's no case law to keep their authority within constitutional limits (compared to, for example, the SEC v. Jarkesy) because a hobbyist isn't going to have the time or money to go through multiple meaningless internal appeals to be able to appeal to an independent court, and anyone who's livelihood depends on it isn't going to risk it.
https://www.imdb.com/title/tt0247745/quotes?item=qt0470809&r...