Canadian but I believe it comes down to whether or not your state has “at will” employment or not - which is basically: it’s my company so I can fire you for whatever I want. I don’t believe California is an at-will employement state.
I am not a lawyer, but my understanding is that "at will" employment allows an employer to fire a worker for no reason (just like an employee can decide to quit without providing a reason) but not for any reason. In particular, you can't fire someone for being black, for being pregnant, for refusing to do something illegal, etc.
This is why there is common advice of "don't tell people why you're firing them" -- as soon as you give an explanation, there's a risk that your words could be twisted into implying that you fired them for a bad (i.e., illegal) reason rather than simply for no reason.
“At-will employment is a term used in U.S. labor law for contractual relationships in which an employee can be dismissed by an employer for any reason (that is, without having to establish "just cause" for termination), and without warning”
https://en.m.wikipedia.org/wiki/At-will_employment
I can fire you for being a fan of the Raiders because I am a fan of the Chargers.
https://www.rocketlawyer.com/article/what-states-are-at-will...