You missed the garbage
Nobody enjoys a mess, whether they are at a store, on an airplane, or on a bus. It’s also a good idea to be mindful of other people, particularly when confined in a small location such as on an airplane with lots of other people.
It appears that the individual either didn’t care or wasn’t really conscious of the mess he was creating. Placing a full tray in the aisle isn’t the answer, as we all know how frustrating it is to have nowhere to put it when flying.