“When thou comest into thy neighbor’s vineyard, and thou mayest eats grapes thy fill at thine own pleasure; but thou shalt not put any in thy vessel.
When thou comest into the standing corn of thy neighbor, then thou mayest pluck the ears with thine hand; but thou shalt not move a sickle unto thy neighbour’s standing corn.” (Deuteronomy 23:24-25)

This finally explains it: why my Mom never thought it was stealing to eat grapes from the bin while at the grocery store.