Well, for one .. our licenses are on the line usually. We're legally obligated to make sure that narcotic prescriptions are dispensed in accordance with medical norms. We're also the stewards of the narco inventory .. so if diversion is occurring, it's our problem.
Also there is that nice feeling you get when you know you're helping keep people out of the spiral of addiction. Too many providers lives have been ruined by it..
I generally am a big advocate for personal privacy, and I could care less if someone is using drugs recreationally in their own homes, but if they're stealing it from my workplace or scamming me to get high, that crosses the line.
Once you've seen a colleague go through diversion->addiction->licensure revocation->poverty->depression and/or suicide ... you generally will understand why it's important, regardless of the legal ramifications of diversion occurring on your watch.
I'll dispense 90ct oxycontin for someone with severe chronic pain without batting an eye .. but dispensing to someone for no real medical purpose, or slacking on diversion vigilance .. that crosses the line. I think a lot of us feel personally responsible for helping to prevent the deleterious effects of such circumstances.