Except that according to his story, she wasn't even over the limit. However by her admission, she signed a document recalling the events, her admission of feeling impaired enough to convict her.
In the US, people are taught to trust the police from childhood and onward. To the average person, it's only the nutters that are screaming about how backwards the police system can be like this, so the usual instinct is to try to be polite and helpful since they don't feel they have done anything wrong. Unfortunately for them, when they do finally end up on the wrong end of a police investigation, they will learn all too well how being helpful is about the worst thing they can do.
So yeah, it was dumb to talk to the police, but most people do not know any better and it is hard to fault them for it. Most people have never even had an encounter with a police officer, let alone been given any reason to personally mistrust them all.