Was this in Ames?
I'm thinking if you were cooperating with an investigation, despite your drinking that
A) There is something else that happened you aren't telling
B) You met a total **** for a cop
The cops in Ames are pretty good about public intox. Seem to let you go as long as your not causing trouble or making a scene.
There is no legal limit for public intox, the balance is whether you are a public hazard. Now with being under age, it is thought that you are a public hazard after drinking regardless of what your doing. If they can show that you were intoxicated, usually that is enough to say that someone underage was a hazard.
If you are of age the officer has to show that you were a danger to yourself or someone else. Even with this, though they can be picky. Things like jaywalking can be enough. Vomiting, you could have alcohol poising not to mention creating a bio hazard. Even losing your balance (doesn't matter if alcohol related) can be used if they find you have a BAC level above normal.
Some of these could be fought if your BAC is low enough, you can cast reasonable doubt that the stumbling was caused by alcohol and not a uneven sidewalk; however, to what point and propose? You pay a lot in lawyer fees for something you can have expunged in a few years.
Last, pleading guilty over not guilty does not guarantee anything, but usually things go better if you are cooperation, especially if your nose is clean in other areas. Just for the record I worked for ISU police for a while and sent my share of public intox's and DUI's to jail.
Edit- as IcSyU says above, normally the police go after people who are by themselves quicker than if you have people with you. Now if everyone in the group is drunk and unable to help each other that goes out the window.