Bottled condiments are usually refilled and reused because by pouring them out, you do not contaminate the contents.
Everything else is pitched out by law. You may not reuse food. Even a basket of untouched bread might have been fondled by someone with dirty hands at the table. Some restaurants have bins for used food and a farmer or someone comes by daily to collect the mess and use it to slop pigs with. There is an enormous amount of waste in the restaurant community, but that is expected. Even in fine restaurants, in the kitchen alone, there is a lot of waste.
Food that has been partially consumed, like a big salad, may not be reused for parts of it has touched peoples lips. Let the health inspectors ever find out that a place is reusing food and there will be major fines!
Now, it might go on in some small hole in the wall restaurants, but they had better not be caught. I know of a BBQ place at the edge of a poor minority section of town that secretly used to supplement its meat supply with freshly killed raccoon or opossum people would bring into the back door. It is no longer in operation.
Some people operate a back yard BBQ business out of their homes, next to a main highway, where they would sell food over their fence and with a place like that, you take your chances as to what they are using for meat.