The Pharisees complained to Jesus about his disciples breaking the Sabbath by plucking and eating heads of grain as they made their way through the fields. If you had asked them why this mattered, they would have replied that they were concerned about the commandments of God. “It’s not lawful,” they said.
But when the people asked Jesus why it was that his disciples did not regularly engage in fasting, they were not asking about commandments or laws, but rather about a widespread, optional religious practice of the day.