Expositor's Bible Commentary Commentary Mark 2:23

"And it came to pass, that he was going on the sabbath day through the grainfields; and his disciples began, as they went, to pluck the ears." — Mark 2:23 (ASV)

The main point at issue here was not the act of harvesting the heads of grain in fields that did not belong to them. Such activity as Jesus and his disciples were involved in was explicitly allowed in the law . What the Pharisees objected to was doing this reaping and threshing on the Sabbath.