You make a good point. The company may fear a lawsuit from others wishing to wear other pins. Or outre pins.
Theoretical lawsuit. Ask yourself if any individual or even a group is going to be able to afford the legal muscle that it would take to go up against Marriott Corporation on such a fine point? They have entire law firms on retainer for this kinda stuff. If it were to happen then Marriott always has the option to settle, and change the policy. I see no reason for Marriott to jump before they get pushed.
The wearing of a flag lapel pin of the host country for a Marriott employee isn't offending anybody or denying others their rights. Not really.