Why do they on the Sabbath what is not lawful? The Pharisees do not blame the disciples of Christ for plucking ears of corn from a field that was not their own, but for violating the Sabbath; as if there had been a precept to this effect, that famishing men ought rather to die than to satisfy their hunger. Now the only reason for keeping the Sabbath was, that the people, by sanctifying themselves to God, might be employed in true and spiritual worship; and next, that, being free from all worldly occupations, they might be more at liberty to attend the holy assemblies. The lawful observation of it, therefore, must have a reference to this object; for the Law ought to be interpreted according to the design of the Legislator. But this shows clearly the malicious and implacable nature of superstition, and particularly the proud and cruel dispositions of hypocrites, when ambition is joined to hatred of the person. It was not the mere affectation of pretended holiness, as I have said, that made the Pharisees so stern and rigorous; but as they expressly wished to carp at every thing that Christ said or did, they could not do otherwise than put a wrong meaning in cases where there was nothing to blame, as usually happens with prejudiced interpreters. The accusation was brought--according to Matthew and Mark--against our Lord, and--according to Luke--against his disciples. But there is no inconsistency here; for the disciples were in all probability so harassed, that the charge was directed chiefly against the Master himself. It is even possible that the Pharisees first wrangled with the disciples, and afterwards with Christ, and that, in the rage of their malice, they blamed him for remaining silent, and permitting his disciples to break the Sabbath.

That which is not lawful - That is, that which they esteemed to be unlawful on the "Sabbath day." It was made lawful by Moses, without any distinction of days, but "they" had denied its lawfulness on the Sabbath. Christ shows them from their own law that it was "not" unlawful.

And the Pharisees said unto him,.... To Christ, the same they said to his disciples, Luke 6:2.
Behold, why do they on the sabbath day, that which is not lawful? see how they pluck the ears of corn and rub them, and eat things, which by the law, especially by the traditions of the elders, were not lawful to be done on the sabbath day; See Gill on Matthew 12:2.