The point is, lying is bad, it rips the fabric of society. Fiction is not lying, it is useful, contextualized nontruth.

Lies are the lubrication of society; you're lied to anywhere between 9-200 times a day.

I see what you did there.

That depends on what the definition of "is" is.

That lying to people in order to make your film feel more realistic is a bad thing to do, even if minor in the grand scheme.

I don't think the intention was in any way to deceive the audience. I personally see it as part of their post-modern filmmaking[0] style.

What personally makes this style interesting to me is that there are usually several layers of meaning and intention. And usually references to other films or common perception or convention.

Even if the intention is to tell the truth or a true story, there's always the bias of the storyteller, the selection of what to tell, and what not to mention etc. So in this case, the Coen brothers re-used the "based on a true story" concept, and even exaggerated it. They re-use the format, but mock it at the same time.

[0] https://en.wikipedia.org/wiki/Postmodernist_film

It doesn't stop being an attempt to deceive just because it's self-aware, ironic, or motivated for other reasons. Likewise, murder during a robbery is still murder even if the primary is theft.