How is it morally questionable? Who hasn't overfilled their cup many times at self-serve drinks counter? Been back for three plates of ice cream at a buffet?

How is it morally questionable? Who hasn't overfilled their cup many times at self-serve drinks counter? Been back for three plates of ice cream at a buffet? It's sounds like a lot of work but completely harmless. Finding loopholes in complicated systems to me seems like a type of arbitrage mor than anything.