> Because the law enforcement officers have to protect their own lives.

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.