Employers in the US often include “morality clauses” that mean they can fire you because they deem you to be harming the reputation of the company due to behavior outside of work.

More importantly than “the rules”, though, US employers can fire you for basically any reason they want and then just lie about it. Nobody is going to force them to be truthful. Not even if they are union busting. The Biden-Harris NLRB, which the president dragged his feet staffing and staffed with wet blankets, has even upheld the Trump NLRB Electrolux decision - and the vast majority of people never get to the point of launching a lawsuit that would be relevant, as it costs tens of thousands of dollars.

If you want power in the workplace you need to organize a union competently and develop capacity real leverage (direct action, community support, naming and shaming).