Why do people get second chances for a DUI?

There is a lot missing here. I believe the ones you have seen that didn't get UCMJ punishment are the ones that got caught out in town. What you don't know is that is coasting them at least $10,000 dollars and Community Service and probation. The other reason is they as in Military risk Double jeopardy and can get in trouble for punishing someone twice. The example is someone gets a DUI out in town after duty on weekend, no other crime and they show up to work on time after the weekend, technically there was no Violation of USMJ and they can only let the civilian court deal with them. On the other hand someone gets a DUI on weekday, can not show up for duty next day, because he or she is in Jail, they can get you for missing duty or UA, still not for DUI, unless on station. Bottom line is every case is different and just because you did not see that person get punished in public per say at Command does not mean they walked away Scott free, things happen like entrées in their service records like letters of reprimand and other things like not recommended for retention or reenlistment. Every case is different.

Commander's discretion. Not all commanders come to the same conclusion on how to discipline their troops. Some will nuke people on the first offense, and others will give someone a second chance, especially if the soldier has otherwise been a solid member of the team.

A soldier should always be punished for a DUI; if there are soldiers receiving a pass for those infractions just know that it is wrong. Now, that doesn't necessarily mean that each soldier deserves to have their career ended for the mistake. Depending on the performance of the Soldier they can still be retained. I personally went to bat for a soldier on his first DUI and we were able to retain him (sadly, he went on to do it again and we dropped the hammer on that instance).