In your example, if he "pulled over", then he was. driving under the influence.
If he was passed out in a parking slot in the bar's parking lot, I would give him the benefit of the doubt.
On the side of the road between the bar and home? Bust him! He and the vehicle weren't dropped there by helicopter...
This is where details matter. I wasn't there, don't know. Maybe he took 4 shots and then drove home before they hit. On the way home, he felt the effects and pulled over to sleep it off. What was his Blood Alcohol content when he pulled over? You don't know, I certainly don't know and the Officer doesn't have a bloody clue. He could have been 0.05, 0.01 or 0.15 - so, on the 'assumption' that he was over the legal limit, he was given a ticket.
I have a philosphical problem with allowing anyone to give tickets, fines or punishments based upon 'assumption'. I have the equipment to be a rapist, is it fair to assume I am one?
You catch the person in the act of committing the crime, whether it's speeding, DUI or failing to stop. You need evidence that a law was broken. For all the officer knew, he could have driven out in the middle of nowhere, sat in his car and drank himself into a stupor - using his car as nothing more than a warm place to sleep.
On the side of the road between the bar and home? Bust him! He and the vehicle weren't dropped there by helicopter...
Pray tell, where on earth is it possible to NOT park between your home and a bar? This is a physical impossibility, as there are more than a few bars on this planet.