Laws that are well written can be applied and enforced consistently. A person can understand the law and conform his conduct accordingly. A police officer can understand the law and enforce the law accordingly.
The statute criminalizing Minor In Possession of alcohol is a perfect example of a poorly written law. It is also one of the main reasons otherwise good college students become labeled criminals, and why so many innocent people come away feeling frustrated with the criminal justice system.
Depending upon your luck, you could be standing 100 feet from a beer and be cited for MIP while another officer only issues citations for those in actual possession of an alcoholic beverage. Some officers believe that if you're at a social event where alcohol is served you are automatically guilty of MIP; others exercise more discretion. The obvious problem with casting such a broad brush is that it criminalizes the social behavior of minors differently than adults. Just as many adults frequent bars and clubs and refrain from drinking alcohol, wouldn't it seem that college students might engage in similar social behavior? Is it unreasonable to assume that just because a person chooses not to drink they may still want to socialize with their friends that do, and that they shouldn't be punished for making a more responsible choice?
By impermissible affixing an illegal motive to group of teenagers who are often not engaged in a criminal act, there are often problems of proof in prosecuting MIP cases.