It doesn't matter what happens years later. It matters that the preacher and his faithful congregation did their part to do the obvious: keep the plague from spreading immediately through obvious means.

You make an interesting point. You bring up, "well somebody somewhere will eventually do something bad, so I might as well as do it right now." There actually is some practical validity to that in some situations.

However, the larger structure of a moral life is, I don't care what others do, I personally will do right.

I knew a soldier once whose job it was to make a certain train not stop in the forest because black market looting of its contents was going on. He forced the engineer to continue to drive, when he stopped for that purpose.

The train made it to its destination with its cargo intact. The soldier was proud of himself.

The cargo was promptly stolen by other people. The soldier knew that.

This did not bother the soldier. He had done HIS job and what other people did was their business.