At a fast food restaurant, that isn't busy. If you're already at a table, can another person just come and sit with you? Can you not ask them to sit elsewhere? If you can't, then can the store management?
I was always taught first come first serve. if you were at a table first, then it was yours. I can understand if the place was busy, but when they're not and there's a million other places to sit, can't they be asked to sit elsewhere?