It does seem like a silly rule - but if I look at it from the perspective of those riding in coach, and from the airline and its personnel, it doesn't seem so silly all of a sudden - soon enough, it will be a chocolate chip cookie, a sundae, a cocktail, maybe an entire meal...

At the very least, it looks like folks in First are sneaking out First Class food to those who have not paid for the privilege - and there's no way for the green-eyed passengers to know the food you were passing was yours, extras, second helpings. If complaints ensue, the FAs have to pick up the pieces.

When the FA told you "The purser won't let you do that; they don't want people bringing food back," you knew perfectly well what it meant, and IMO it was disingenuous to use these words to get off the hook - glad it worked for you, and while I do think a report is a bit too far - but you were warned.