So maybe the lesson of this anecdote is not people are 'simple minded' (read: stupid), but that people are not computers and bring a complex bundle of conscious and unconscious motivations to every situation that will influence their decisions.
I'm not saying you're stupid unless you're robot tier 100% impartial, but if you're just letting your gut feeling take you all the way, then yes, you are.
You do have a point if, as stated, it was proven that the driver was indeed driving drunk, but just couldn't be proven to be over the limit, in which case the juror abused her position to enforce what she thought the law should be rather than what it actually was (but rationally so).
Obviously this isn't the best situation.
It sucks that in the gun/murder example it would ruin someone's life, and I would advise them to be more thoughtful if I could, but there's no accounting for taste.