From what I've seen lately, Doc likes to get a ton of drugs, multiple cycles, really high crime. Then he just DCs, comes back after a long time and everyone has forgotten about crime, and just sells as cop. (I believe). I haven't been able to rob him in a while despite him being very obvious with him crime rates, it's also very hard to catch him on demo doing this, because all the demo will show is him DCing, which removes his bug, then reconnecting after a LONG time, and playing as cop.

I really wish they would disable cop drug sales, it's heavily exploited server-wide.