The police were there because the restaurant called and asked that the couple be removed and given a trespass warning, meaning they didn't want the couple back. The restaurant has the right to kick out any patron for any reason other than illegal discrimination (i.e. because of their race, color, or religion under federal law and their race, color, national origin, sex, pregnancy, handicap, familial status, or religion under Florida law). The restaurant need not give a reason why either to the police or to the patrons. Nor do the police need to give a reason to the patrons, assuming they were even told what it was by the restaurant. Usually signing the trespass order only is an acknowledgement that the person has been advised of the trespass order. Not signing it, however, does not make the trespass order invalid. They were told they can't come back and thus if they do they face the risk of arrest for trespass.
Now I'm going to guess that there is more to this story. Restaurants are in the business of making money and they do that by serving patrons, not having them chucked out and told never to come back. So that suggests to me that one or both of them did something that the restaurant staff did not like. As quinsy said, they likely have some idea of what that was. Knowing what that was would help shed light on the problem. But even if they truly don't know the reason, they still are stuck with the situation that they are banned from the place.
