Drunk people wandering amongst traffic in the street, or the drunk ones wandering down the sidewalk and repeatedly falling of the curb into the street would seem to pose as much, if not more, danger than a stationary, passed out person.
I suspect it isn't so much that the law is way more aggresively enforced over VEISHEA weekend, as much as there are just way more drunk people wandering around posing danger...
A little bit of both is the likely explanation.
I've heard of a lot of people getting public intox's for just simply walking alone on the sidewalk. Although 99% of people pose no danger to themselves by doing this, you do have your Abel's and John Lacina's.
About half the people I know who got PIs got them in this fashion. Those always seem to get written up on Thursday nights when there is no drunk bus, but a lot of people want to go the bars, and don't want to drive home.
The other half of the folks I know who got them deserved it.