There should a designated visitors gallery on the airport, because it is a public place for the public. Any public utility has the responsibility to take care of the visitors irrespective of whether they avail their services or not.

Take the example of City Centre, Villagio Mall, Bus station etc.

The airport officials can put up a fee on the people waiting in the waiting/visitors gallery. This is applicable and successful in several far eastern airports and railway stations, where, even if you are not a traveler, you have to buy a "visitor ticket".