That is insanely stupid, if they're calling it an actual DUI. If you're not Under the Influence... how can it possibly be a DUI?
Ugh... Make up another regulation about open containers, but don't call it a DUI, because it's not, if you're sober.
If someone opened a container, in the backseat, while you're driving, perhaps without even your knowledge, you can get a DUI. Insane.