From my reading of the circumstances involved in that incident, it was not a case of a criminal just coming up to a police officer and deciding to take his gun away from him, but a situation where conflict had already been generated by the officer attempting to arrest / detain him and it escalated. You also cannot compare what happens in the outside world with what happens in prison. In the outside world, they have an incentive to not get caught (or shot), whereas in prison, they might figure that they have nothing to lose.Felons do intentionally attack police. Brown attacked Wilson. Most seek a vulnerable victim.