Separating the trimmings from the rest of the waste isn’t the only thing that requires effort. The management doesn’t want to give ordinary employees the authority to just give stuff away, which makes sense. Even if it isn’t a problem in this specific case, it can be a problem because employees won’t always be knowledgeable or honest. Having management review what is being given away involves overhead, and deciding how much to charge you because of that overhead involves more overhead. I probably wouldn’t bother with all that if I ran the supermarket unless I really hated throwing things out, because you probably wouldn’t be willing to pay enough to make it worth my time.