Why do the disciples steal corn on the sabbath?

Mark 2:23 And it came to pass, that he went through the corn fields on the sabbath day; and his disciples began, as they went, to pluck the ears of corn. Matt 12:1 At that time Jesus went on the sabbath day through the corn; and his disciples were an hungred, and began to pluck the ears of corn, and to eat. Luke 6:1 And it came to pass on the second sabbath after the first, that he went through the corn fields; and his disciples plucked the ears of corn, and did eat, rubbing them in their hands.
Mark 2:24 And the Pharisees said unto him, Behold, why do they on the sabbath day that which is not lawful [1]? Matt 12:2 But when the Pharisees saw it, they said unto him, Behold, thy disciples do that which is not lawful to do upon the sabbath day [1]. Luke 6:2 And certain of the Pharisees said unto them, Why do ye that which is not lawful to do on the sabbath days [1]?
Mark 2:25 And he said unto them, Have ye never read what David did, when he had need, and was an hungred, he, and they that were with him? Matt 12:3 But he said unto them, Have ye not read what David did, when he was an hungred, and they that were with him; Luke 6:3 And Jesus answering them said, Have ye not read so much as this, what David did, when himself was an hungred, and they which were with him;
Mark 2:26 How he went into the house of God in the days of Abiathar the high priest, and did eat the shewbread, which is not lawful to eat but for the priests, and gave also to them which were with him? [2] Matt 12:4 How he entered into the house of God, and did eat the shewbread, which was not lawful for him to eat, neither for them which were with him, but only for the priests? Luke 6:4 How he went into the house of God, and did take and eat the shewbread, and gave also to them that were with him; which it is not lawful to eat but for the priests alone?

Matt 12:5 Or have ye not read in the law, how that on the sabbath days the priests in the temple profane the sabbath, and are blameless? [3]

Mark 2:27 And he said unto them, The sabbath was made for man, and not man for the sabbath: 28 Therefore the Son of man is Lord also of the sabbath. Matt 12:6 But I say unto you, That in this place is one greater than the temple. 7 But if ye had known what this meaneth, I will have mercy, and not sacrifice, ye would not have condemned the guiltless. 8 For the Son of man is Lord even of the sabbath day. Luke 6:5 And he said unto them, That the Son of man is Lord also of the sabbath.

[1]Exodus 34:21 Six days thou shalt work, but on the seventh day thou shalt rest: in earing time and in harvest thou shalt rest.

[2]1 Samuel 21:5 And David answered the priest, and said unto him, Of a truth women have been kept from us about these three days, since I came out, and the vessels of the young men are holy, and the bread is in a manner common, yea, though it were sanctified this day in the vessel. 6 So the priest gave him hallowed bread: for there was no bread there but the shewbread, that was taken from before the Lord, to put hot bread in the day when it was taken away. Note that in 1 Samuel 21:1 the priest's name is listed as Ahimelech as opposed to Mark's Abiathar.

[3]This refers to the priests being able to perform ceremonies on the Sabbath. For example, Numbers 28:9 And on the sabbath day two lambs of the first year without spot, and two tenth deals of flour for a meat offering, mingled with oil, and the drink offering thereof: 10 This is the burnt offering of every sabbath, beside the continual burnt offering, and his drink offering. (New Oxford Annotated Bible, p. 1185).