Maybe they are the type of people who don't want to punish you if you haven't hurt anyone.
DUI laws are pre-emptive punishment for potentially harmful behavior. If you actually do hurt someone, there will be additional charges, not just DUI.
If it came down to guns .vs. butter, I think I'd rather save punishment for people who cause harm, instead of exhausting my resources looking for people who've made bad decisions that have a high chance of causing harm. But apparently we have limitless resources to devote to punishments; after all, we can always rip more money out of public assistance and education budgets, right? So I guess everything's OK.
NOTE: The above is an exercise in open-mindedness, rather than an attempt to defend Montana or her citizens, or any sort of plea for repeal of DUI laws. Please do not send the police to my house to brutally punish me for my thoughtcrimes!