Recovered drug addicts can never fully be trusted because even after a drug addict has recovered, the capacity for addiction does not leave them. No matter how long someone has been recovered from their addiction, the possibility of relapse exists. Between 40% and 60% of recovered addicts eventually relapse, and if relapse occurs, then addicts will quickly resort to deception in order to feed their habit.  Whether it is due to embarrassment, wanting to hide the addiction, or a desire to acquire money for drugs, addicts inevitably resort to lying. Even if addicts actually want to kick the addiction, their minds are twisted by the addiction in a way that causes them to distrust others, and that, in turn, makes them act deceptively toward others. Just as practicing any skill will cause improvements, once a drug addict begins lying, they get better and better at it. Lying creates more and more complex defense mechanisms which make it easier to continue lying. 
Since only 40% to 60% of recovered drug addicts relapse, it doesn't make sense to treat all recovered drug addicts as untrustworthy. That would unjustly condemn about half of the population recovered drug addicts who are potentially trustworthy. Additionally, treating recovered drug addicts as if they are untrustworthy will likely encourage some of them to relapse due to their isolation and the negative emotions directed at them.