Question: Why were they even kidnapping people in the first place? The ransom seemed like it wasn't planned and they only did it when they discovered that Jeff was rich. So if there is no ransom usually, why even kidnap couples to begin with? If it's just to rob them, then that doesn't seem very lucrative for the risk.
Question: When the kidnappers leave the barn after putting Amy in the freezer, why doesn't Jeff try harder to pull the latch off the door and free his wife? If I were in his situation, I would be desperate to free her and run away so we could call police. He tries a few times to pry the bar off, but eventually gives up and runs into the house. This doesn't seem to be a very wise strategy to adopt.
Answer: There was always a ransom, or at least that was always the intent. Anyone that didn't have money the kidnappers would just eliminate them. The kidnappers MO was to take a couple and force one of them to empty their bank accounts. Earl makes it clear that the difference in Jeff's case is that they were unable to kidnap the pair at the same time. For whatever reason, Jeff refused to get in the truck with Red, which caused the kidnappers to alter their plans slightly. Earl also gives some insight into how the group chooses their victims. He tells Jeff that he was targeted because he was driving a brand new SUV, which indicated he must have a fair amount of money in the bank. The kidnappers care very little about being caught, they have obviously committed many murders without being found out. They seem to leave very little evidence of their crimes for the police to find. Red is always several steps ahead of the police and is brazen enough to kidnap a man's wife and say directly to him in front of a police officer that the man must have him mistaken for someone else.
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