Because the law enforcement officers have to protect their own lives. This is America, where anyone could be carrying an implement that requires two seconds to kill.
Against a teenage girl who may have gasp bought some beer?
How about:
"Excuse me, miss. We suspect you have illegaly purchased alcohol. May we inspect your car, please?"
Scenario 1: "Oh! Ok." Car is inspected. "Oh it's just water. Sorry about the trouble, have a good day!"
Scenario 2: Girl tries to escape and drives off. Note personalia and license number. File charges as usual.
There is no realistic scenario in which they need to threaten her on her life and assault her car because of a suspected 12-pack of beer.
According to this (admittedly outdated) research paper [1], the high-end estimate of traffic stops resulting in the homicide of the police officer is 1:20.1 Million. This doesn't really sound like a traditional traffic stop - more like a sting (something that I couldn't readily find data for).
[1] http://blog.lib.umn.edu/jbs/Criminal%20Procedure%20in%20Amer... (Warning... PDF)
but then it hit me how much this is sad, USA seems 100% psychotic to me these days... I honestly feel like stopping following US-related news because it's all so depressing goddamn
I think that if police are worried about being shot at when they approach under-age people buying alcohol the police might want to change their hyper-aggressive policing style.
Guns must be a measure of last resort. When you pull your weapon you must have the intent to kill someone. To have the intent to kill someone you must have reasonable fear that they pose a risk to you.
Under 21 year old girl buying "beer" doesn't feel to me like a deadly threat to my life.
Those cops grew up watching this crap, they know what a 'good' cop is supposed to do.
But I wonder if it is worth the time, money and effort of 6 police officers to prevent kids (even though she is adult) from drinking ? Drunk driving is one thing, but drinking ?
I agree with you that this is a big waste of resources, though.