Police jumping to conclusions after just seeing a boy with a keg should not surprise anyone. After dealing with some of the worst people in society day after day, most police officers become jaded and expect the worse from practically everyone they meet. Police tend to think in terms of "us" vs "them" when it comes to citizens.
The ability to make a decision on incomplete information is a survival mechanism. We use past experience to fill in the current blanks. With police, seeing a teen with a keg is most often going to involve a violation of the alcohol laws. The one time out of ten? fifty? that the teen has an alcohol-free beverage in the keg is the exception rather than the rule.
The police are notorious for jumping to conclusions. They’ve “accidentally” gunned down many an unarmed teenager because of a conclusion that such a person had a gun. I know it is probably tough being a police officer in these violent times, but not everyone in the world is holding something bad.
No, the police generally do not jump to conclusions, however, this can happen once in a while. Most teenagers would not have a keg of root beer so the police were reasonable in assuming that the keg contained actual beer. The police can make mistakes some times so long as the person is released unharmed.