Jamieson, Fausset and Brown on Deuteronomy 23:24-25

24-25. When thou comest into thy neighbour's vineyard, then thou mayest eat grapes thy fill at thine own pleasure--Vineyards, like cornfields mentioned in the next verse [De 23:25], were often unenclosed. In vine-growing countries grapes are amazingly cheap; and we need not wonder, therefore, that all within reach of a person's arm, was free; the quantity plucked was a loss never felt by the proprietor, and it was a kindly privilege afforded to the poor and wayfaring man.