A question Michael: how many of those LEOs that were disarmed were done so by a person to whom they made the first contact? What I mean is, was the officer attempting an arrest or enforcement of a police action? If so, the perp was in a 'fight for his life' situation. By this I mean they did not walk up to the officer and attempt to take his weapon, they only did so in order to maintain their livelihood and freedom.
My guess is that the likelihood of a perp specifically picking out a LEO to get his gun or to commit his crime is minimal. In the same way it seems unlikely that a perp will attack me because I have a gun. Given the choice, I would think most perps would look elsewhere before attempting a robbery in a store with an armed person standing nearby.
I don't know if this directly pertains to your question or not, but in 1971 two NYPD officers were shot in the back from ambush and their service revolvers were taken. The perps were members of what was called the Black Liberation Army, and while I doubt that this was the primary cause of the murders of Waverly Jones and Joseph Piagentini, the guns were taken. Gregory Foster and Rocco Laurie, also of the NYPD were murdered in similar fashion a year or so earlier and I believe, although I am not certain, that their revolvers were taken as well. So, it does happen; not often, but it does happen.