In the opening paragraphs of the story, Sam describes the town they have chosen to attempt the kidnapping as such: 

There was a town down there, as flat as a flannel-cake, and called Summit, of course. It contained inhabitants of as undeleterious and self-satisfied a class of peasantry as ever clustered around a Maypole. 

First of all, the town is called "Summit" which suggests something high, like the top of a mountain. But the town is "as flat as a flannel-cake." Sam gets the impression that these people are so simple-minded that they don't even know the proper way to name their own town. The citizens are "undeleterious" and this means they are harmless. They are "self-satisfied" peasants. This suggests that they are comfortable with a simplistic, rural lifestyle. In other words, they are not cultured, probably not that smart, and they are okay with that. So, these rural people ought to be simpleminded and unlikely to challenge them in the kidnapping attempt. 

Philoprogenitiveness, says we, is strong in semi-rural 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. We knew that Summit couldn’t get after us with anything stronger than constables and, maybe, some lackadaisical bloodhounds and a diatribe or two in the Weekly Farmers’ Budget. So, it looked good. 

Sam and Bill concluded that, in rural communities, people must love their children (philoprogenitiveness), thus making it likely that the parents of the kidnapped child will be more than willing to provide a ransom. It also makes sense to choose a rural setting because there is less likelihood of major press getting involved. And they also suppose that the law enforcement in such a small town is probably not that extensive or even used to dealing with this sort of thing. 

These are all logical conclusions based upon their assumptions, but some of their assumptions turn out to be wrong.