Why do people always conflate legality and morality. A wedding guest might not be doing anything illegal eating/taking so much food/drinks that there is nothing

Why do people always conflate legality and morality. A wedding guest might not be doing anything illegal eating/taking so much food/drinks that there is nothing left for anyone else, but I'm pretty certain many would make the decision to not invite them again, even without giving them a trial. If we talk about its legal, whose laws should we even apply?