The woodpile has not burned yet. It is only possible that it does harm. Why may someone step in anyway?

Because it is also only chance that it does no harm. No one can rely on that. Here lies the wrong in such actions. They expose others to a danger.

In the end, the right of the police to punish rests on this.

Summary: A harm is only possible. But it is just as much a matter of chance that it does not happen. Here lies the wrong, and the right of the police to punish rests on this.