It's a slippery slope. Lets say you and I are in coach class of a flight. You ask for a first class meal and give the FA $20. You know what, I'm hungry, here's another $20 FA, bring me a meal too. "Sorry, MBD got the last one". So now I throw a fit and start scratching and threatening lawsuits. Why are you entitled to a meal and I'm not?
Of course, the airline could sell left over meals if they so desire. It's a business decision. If you don't like their decisions/policies, don't fly that airline.