Ah. That isn't expression. That's an action. Speech (expression) is protected. Action is not unless it is defined as speech or is otherwise determined to be a "right". Refusing service is not speech.
Actually, it's a lack of action, but that aside if a person can't decline a service that goes against their conscience, have we lost the right to free expression?
"Things should be made as simple as possible, but not one bit simpler." ~ Albert Einstein