According to the Pharisees, the disciples reaped a crop. They threshed it by rubbing the berries in their hands and breaking the hulls off. Then they winnowed it by blowing the hulls away. By doing so, they were guilty of preparing a meal. This was actually a high holy day, very likely one of the holy days of the Days of Unleavened Bread.
Consider the disciples' motivation for what they did. First, they were hungry. Second, they were itinerate, using "shoe leather express," traveling with Jesus as a part of His entourage. He instructed them, giving them examples of His way of life, all along the way. He Himself said that He had no place to lay His head. They had, therefore, no place to prepare a meal. They did not have homes that they could readily return to.
These were strong, young men, probably in their twenties or early thirties (about the same age as Jesus), so they could have fasted without damage. But, because it was the Sabbath, Jesus deliberately drew attention to one of the Sabbath's main purposes: It is a day of mercy and not a day of sacrifice.
Christ's justification comes from I Samuel 21:1-6. He reasoned that, if it was all right for David to allay his hunger under an unusual circumstance by eating bread that had been consecrated for holy use, His disciples could provide for their needs in this manner. (The showbread was put into the Tabernacle on the table, and it sat there during the entire week. Then, every Sabbath it was exchanged for new bread. David ate the week-old bread that had just been exchanged for the new.)
So what is He saying? The Sabbath is a day of mercy. And if one can rightly, lawfully use "holy bread" to do something that, according to the letter of the law, was illegal, then it was also legitimate for the disciples to provide for their needs also in an unusual circumstance.
The emphasis here is on the word unusual. How frequently was David fleeing for his life and finding himself hungry? It did happen, at least this one time, but it did not happen every Sabbath. Maybe in David's lifetime something like this occurred a few times, but even for a man of war like David, it did not happen all that frequently.
The overall lesson, however, is that it is not the intention of God's law to deprive anybody of good things. The intent of God's law is to ensure life. If the need arises, one should not feel conscience-stricken to use the Sabbath in a way that would not "normally" be lawful. Christ admitted that what David did was not "normally" lawful. Neither was what the disciples were doing "normally lawful," except for the extenuating circumstance.
In this case then, they were blameless because a larger obligation overruled the letter of the law. The larger obligation was to be merciful. The letter of the law said that they could not have that bread. The larger obligation said that it was more important to eat than it was to fast (to sacrifice eating). Holy bread, or holy time (the Sabbath), can be used exceptionally in order to sustain life.