There's gray here; weapon 'systems' might include items such as standard issue rifles, and they could be targeted, as Lewis Page (from the Register) notes:
“If people or property isn't a military objective, we don't target it. It might be destroyed as collateral damage, but we don't target it. Thus in many situations, we could target the individual holding the gun and/or the gun and legally there's no difference.”
Which seems to suggest that a robot could decide, under Mr Canning's rules, to target a weapon system such as an AK47 for destruction on its own initiative, requiring no permission from a human. If the person holding it was thereby killed, that would be collateral damage and the killer droid would be in the clear. Effectively the robot is allowed to disarm enemies by prying their guns from their cold dead hands.
Rough Type wrote this up as well. Why do always have to go for all the marbles? Can we just experiment with rights for the Roomba for a while, before we start getting into the Ape-killing scenario. I'm sure Dr. Zaius is turning over in his grave.