There use to be this ideal of "innocent until proven guilty". What makes you trust this woman so much?
The woman's word didn't convict them. It amounted to reasonable suspicion to pull a vehicle over, that's it. Your ignoring a big chain of events.
It could have been a female Federal agent wanting to create a parallel construction of evidence.
Its conceivable. Do you have any evidence for this beyond a mere statement of the possibility that it might be the case?
An anonymous call, by itself, warrants just the smallest of investigations -- in this case, the law enforcement official followed the vehicle and did not see any evidence of wrong doing. That should have been the end of it.
Because pulling the driver over and asking if he was drunk is an "exceptionally deep investigation"?
The woman apparently did not want to press charges
Press charges for WHAT?
or even testify since it is assumed she refused to give her name.
Not wanting to get involved is pretty normal. If I saw a drunk I wouldn't want to get dragged into court, have my character, recollections, and life torn apart by cross examination, my time wasted etc. And if the police catch up to him because of my tip, and notice he's drunk then they can take care of him without my help.
I suspect in this case, however, you'd need to have the caller identified since if the run-off-the-road case was taken to trial, the defendant has a right to cross-examine witnesses.
Only if he was charged with running the other driver off the road. If he's to be charged with drunk driving, the fact that he's impaired when pulled over is more than sufficient.
I do think they overstepped what was appropriate for the situation.
Pulling him over to see if he's drunk is inappropriate after a report he was driving badly. FWIW Drunk drivers rarely drive erratically. Their reactions and awareness is impaired; so unless something unexpected happens they'll probably appear to be driving just fine.