The way it's suppose to work is once they are activated with that personality, it does an inventory check for the "Subway Token". If you have one, it will take the token out of your inventory, then attack everything else in site but spare you.
I don't know whether it's a one time event, or would require more tokens upon revisiting the location later.
However, I've heard a lot about it being bugged at the moment and not working right. If that is the case, then a patch is set to be released this upcoming week. Hopefully that will be one of the fixes.
My recommendation for the robot is to set the personality to Law Enforcement, and keep your weapon holstered. Whenever enemies come out, run behind him. He will destroy whatever is around. If at any point he is destroyed, then step in and finish off the enemies. This way he never attacks you unless you draw your weapon, and it will save you ammo since he's killing off enemies.
When one is up against raiders they seem to be able to handle a few. If your lucky 5-10 at the most.
Ghouls they seem to be handle around the same number. Different people may have different results.
I have new information based off of new experience, and research.
I found some information at the following links:
I have tested this. You have to have a Subway Token in your inventory once the protectron is activated with this personality. Once the coin is requested, you have exactly 8 seconds to comply. In order to do this you need to press the action button immediately after he requests the coin, within 8 seconds.
You can pick pocket the coin back from him, but the next time he see's you, he will request it again. This information can all be backed up by the above URL, and a YouTube video showing this in action.
I have also verified this from personal experience within my own game as well.