On the other hand we don’t know, and can’t know, how many events their current practices have prevented. It may be that 90% of the time dropping in and showing the parents what their kid has been up to is enough to prevent something from happening. The question is what do you want to do to that 90% that could’ve been avoided anyway to get more of that 10% that is going to proceed.
Edit: I personally know someone whose parents got a visit from the FBI back in the day. I can say with certainty that they were never going to shoot anything and were just idiot kids who made a bad joke too soon after Columbine. So not every FBI interaction actually prevents anything to begin with. That’s the challenge with gauging the impact of preventative measures.