The father's response surprises the kidnappers because they expect him to love his child and want him back. In fact, the kidnappers rely on this for their scheme to earn money:
Philoprogenitiveness, says we, is strong in semirural communities; therefore, and for other reasons, a kidnapping project ought to do better there than in the radius of newspapers that send reporters out in plain clothes to stir up talk about such things.
It is thus definitely surprising to Sam and Bill when the father of the kidnapped child gives them a counter-proposal: they are to bring the child back to the father, and, in lieu of claiming the ransom, they are to pay the father 250 dollars themselves to take him off their hands. Bill, the more battered between the pair, hastens to bring back the child and pay the ransom to the father. He then runs away as fast as he can from the child. It's certainly an unusual way to respond to a kidnapping!