This discussion is moving into silly areas.
Of course there are legal and practical limitations on a Captain's authority, but it is his duty to take steps to preserve the safety of his airplane and passengers. Laws and regulations, sops and company stipulations cannot hope to cover all situations. Obviously there's a distinction between decisions made on the ground and in the air.
You can't trust Captains to make the right decision all the time but in my experience ground staff are generally less trained, experienced and frankly disinterested in the safety of the flight to make the best decision. After all, they're not the ones who'll be airborne in an aluminium can with wings when the worse case scenario comes calling.
I had a dispatcher in Warsaw once tell me that it was unnecessary to anti-ice the airplane. She put up quite a fight because there was no precipation at that moment in time, clearly regarding my authority as Captain to be subservient to her couple of years dispatching flights. She only relented when the snow I calmly predicted started falling: suddenly anti-icing was needed. Arguing was such a waste of everyone's time and energy. Even if I hadn't been right, what authority did she have to deny the request?