There is a place in my town that reportedly would collect the shells from touristy lobster dinners and use them for "bisque" . So gross.It really depends on the food matter right and how long it has been exposed. For example if you have bread that has been out I see no issue with preparing breadcrumbs from it. If you have say a cold chicken salad appetizer then the risk is too high so no. If it is baked potatoes I would keep them and use them in a soup or meatloaf myself.
Can you be more specific about what type of food is being reused?
In my cooking history I once worked at a restaurant that served chips and salsa and the restaurant re-used the chips that were not eaten. Yuck.
I currently work at a private school. As I recall once food reaches a buffet line it is dead and cannot be reused. Yet this is a common practice here. I've been having a difficult time finding if this is a violation or not.