re: Why do a large portion of Americans believe headlines even without reading the article?

I can tell you as a former newspaper man, and this was especially true in my day, the folks who pen the headlines are not the ones who write the articles and they are just trying to fit a headline in that fits in the space provided.

And if they read past the second paragraph to try to get an idea for the tagline or title, you're lucky.

Lots of very misleading headlines on articles.