In the state of Michigan, a minor (the under 21 kind, not the under 18 kind) could be forced, without a warrant, to take a breathalyzer test, even if they aren't driving in a car. Being in a party where there are adults who are under 21, can result in forced testing. (By the way, our society has to decide either 18-20 year olds are minors or not).
Fortunately, a federal court ruled against these violations of the 4th Amendment. Before this ruling, there were cases such as a young woman who at a party, left her purse there, and was woken up by police in the dead of night to take a breathalyzer test, since the police got her I.D. from her purse. Simply being in a party with possible drinking was enough for police to force you to take a test after waking you up.
Heck, if you are not even suspected of being a minor and you are not driving in a car, you could be arrested as a passenger. This is simply, stupid public policy. We as a society want intoxicated individuals to not drive home, but if being in a car is considered public intoxication and grounds for arrest, than the incentive not to drive home while drunk, is lost. Any police officer who arrests a passenger for being intoxicated, and nothing more, is simply doing harm to the cause of curbing drunk driving.
Indeed, this is what happened to Christina Aguilera, who was not driving, but a passenger when arrested for intoxication. Unfortunately, the impression in the minds of many not following the details of this case, is that she herself was driving. Tell me, what is the harm and threat of an intoxicated passenger? None. Who are they bothering while in a passenger seat? Nobody.
Laws should and must be based on stopping one person from harming another, not just on the whims of lawmakers and police. Finally, in one case, an intoxicated driver with her infant was given a warning and not arrested, but her passenger was arrested for public intoxication and was refused, her request, to be given a breathalyzer. So, these laws not only don't make sense, they aren't even applied evenly.