Many countries quarentine certain inbound foods, and that includes any uneaten food served on the plane. I'm assuming that the USA does it, since you're not allowed to bring any produce or red meat into the country. I'd imagine it's easier to just burn everything arriving rather than pic through the rubbish. I don't know about the prepackaged items, such as the snack boxes though.

I'll defer the answer regarding domestic flights to someone else.