I’m intimately acquainted with recycling protocols. The recycle center sends the sorted plastic, paper and carboard, metal and glass to different companies that use it to make new and useful items. They really need their donations to be clean for several reasons.
First, that’s what the folks who accept recycled raw materials demand. They require their raw material to be free from food, dirt, etc. Otherwise they can’t make it into new and useful stuff.
Second, food residue grows mold and attracts bugs, and other vermin, besides smelling ghastly. Again, these companies that accept the raw material can’t use contaminated product.
If you think about it, the idea of recycling “clean” plastic, carboard, paper and metal should be obvious, but it doesn’t seem to get through to many people in my country. The Japanese have very specific standards for recycling, and they enforce it.