Just to pipe in here, there is really nothing "offical" that says that the tables are only for the patrons of that particular counter service restaurant, are there? (If there are, my apologies.)
So, technically, even someone who is not dining there has a right to sit down at a table to get out of the sun and rest for a few minutes.
Likewise, when someone is done eating, they have a right to sit there a bit longer to digest before hitting the rides/shows/etc. again. Even if someone else is looking for a table.
I have always gotten a table for our family while DH gets our food. And honestly, if the place is as crowded as you're talking about, by the time we find a table, get settled, wash hands with wipes, get napkins, etc., DH is there with the tray full of food. And if I end up sitting there a few minutes waiting for him, well, I will.
Flame away if you like. It won't bother me, becuase I don't have a problem with this practice, whether it's me or if I see someone else doing it. And FTR, I consider myself a very polite and helpful person. I'm the kind of driver who will stop and let you in the lane when it's backed up, I'll hold the door for anyone behind me, etc.