II. The religious practice misunderstood (v. 24)
In verse 24 it is clear that the Pharisees had some very strong opinions on how the Sabbath was to be observed. Mark tells us that as the disciples were munching down heads of grain “the Pharisees were saying to him, ‘Look, why are they doing what is not lawful on the Sabbath?'” The first thing that jumps out at me about this verse is where did these guys come from? Where they army crawling through the fields waiting for some slip up, or were they awkwardly following Jesus at a distance? Either way, I think we are same to deduce that the Pharisees continually followed Jesus around badgering Him. This time the Pharisees were badgering Jesus about the conduct of His disciples on the Sabbath.
More than anyone else the Pharisees worked hard to figure out exactly how to practice their beliefs, particularly when it came to the observance of the Sabbath. Their solution to how to observe Sabbath was “to leave nothing to chance, but by legislating for every circumstance to protect the faithful from ever breaking the prohibition on sabbath work.” They had all kinds of extra rules, regulations, and traditions for keeping the Sabbath. In fact, if you followed the traditions of Pharisees you better not dislocate your foot on the Sabbath. They claimed that it was not necessary to have your foot fixed and so it was illegal to receive medical attention for a dislocated foot on the Sabbath. D. Edmond Hiebert further explains these regulations:
To ensure observance of the Sabbath law, the scribes had enumerated the main clauses of work: forty save one, among them reaping, threshing, and winnowing. But in their eagerness to ‘fence’ the law against violations, they had gone to extremes in stipulating acts that were regarded as work. Thus the pedantic Pharisees regarded plucking of heads as reaping, the rubbing out of the grain as threshing, and perhaps the blowing away of the chaff as winnowing. The disciples were guilty of working on the Sabbath!
This is why the Pharisees accused Jesus’ disciples of breaking the law. But in their zeal to uphold their religious practices they failed to recognize that Scripture makes a distinction between reaping and what the disciples were doing. Remember, according to Deuteronomy 23:25 were not reaping; had they been reaping it would have broken this law. This means that the Pharisees were wrong when they accuses the disciples, and by implication Jesus, of breaking the law. But even more important than this is the fact that the Pharisees completely misunderstood the Sabbath, and they misunderstood who Jesus was. These two key misunderstandings were why a conflict arose between Jesus and the Pharisees. In verse 25-28 Jesus deals with both of these issues.
 D. Edmond Hiebert, The Gospel of Mark: An Expositional Commentary (Greenvile, SC: Bob Jones University Press, 1994), 80-81.