I really hate to fall on "what if?" arguments, but I on the local radio here they have a lawyer ( shoutout to russmanlaw.com ) on to answer listener's legal questions, and one time one posed a question very much like the situation here. His response was, "what if the person you warn is actually doing something bad? "
What if they are driving drunk and the flash snaps a tiny bit of sense into them, long enough to slow down a bit and drive straight, but as soon as they pass the cop, are off and weaving again.
What if there's an Amber Alert in the area, and the guy you flash those beams to has a kid in the trunk? What if you prevented him from getting pulled over for speeding, when the cops would've seen or heard...
I know it's logically fallacious and one-in-a-million, but I couldn't live with myself if I found out something bad happened because of it. The only downside of not warning oncoming drivers is that maybe they get ticketed.
That I can live with.