The penalty for being wrong was none, and the story was quite entertaining. Why not hold it as true?

Sometimes there is inertia from fooling someone credible and influential. Once you have them you sail through everyone else. If the person later discovered your fraud they aren't likely to expose it because it would ruin their credibility.

I have seen banks get taken by people and just sweep shit under the rug.

In case of a good story, I suppose Spielberg couldn't care less if that story is entirely based on facts or mere fiction (and a good story it is).