What if one of these gets out of control/hacked/malfunctions? This is one more reason we need weapons to protect ourselves.
Hacking is an interesting one, only because current laws assume the perpetrator must be in proximity to the crime.
However, this robot is almost certainly not autonomous with the operator hidden behind the camera. Because it's not autonomous, it's going to be difficult to teleoperate over the internet. Telerobots are slow and not very capable[0]. With lag, it's going to be difficult to stab someone with a knife much less pick up a knife.
However, even slow telerobots present interesting legal issues. One might hack a telerobot in the home of some people who are on vacation and use it to unlock the doors so some guy can go in and steal stuff.
http://spectrum.ieee.org/automaton/robotics/robotics-softwar...
If you are seriously worrying about such situation you should also worry that robot can pick up the weapon.
Sorry, I was lucky enough to be born in a country where I'm allowed to defend myself. And if a robot or a person starts raping me, I'm not going to let them finish until they wear out.