You're right...you just keep posting the same thing. Your basic point is that if the cops can't easily decided who is actually in possession of a controlled substance then everyone in the general area should be charged. We all got that point the first time you posted it.
The point still stands however that not every person in a grouping of people is responsible for what others do. Sure, sometimes people can get railroaded when the cops are lazy and the judge just rubber stamps it, but that doesn't make it right.
In many situations, especially with teen parties at a private residence, many people at the party may be totally unaware of what another is drinking. If you've ever been to a college football game as a student this should be crystal clear. I saw many students sitting near me drinking alcohol that they snuck in...that doesn't make everyone in the student section guilty of possession.
The same thing happens at teen parties...a few people bring in their alcohol and others are unaware or or uninvolved with what they are drinking.
In regards to your alcohol in the car scenario, if one beer was in the car and one person was drinking it, it is truly doubtful that others would be charged.
Sometimes, the Bill of Rights trumps the cops' convenience. If a cop can't ascertain who truly committed a crime, then no one should be charged. Why should people that have committed no crime be punished just so the cops can take short cuts?