If she has a gun, she can shoot at least one police officer. And as a police officer, you don't want to be the one getting hit. So human survival instinct says that you will try to protect yourself (even well trained professional could fail at times to control their basic reaction).

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 ?

But by that logic, police should just pull a gun on everyone they ever see, because the person could have gun themselves.

I agree with you that this is a big waste of resources, though.

If I recall correctly, most humans freeze, not shoot. The natural reaction is to either flee or freeze. I think policemen are specifically trained to take the gun out of the holster and press the trigger in such situations.

Is preventing an adult from drinking alcohol really a constructive use of 6 polices officers?