What has that to do with ethics? That's simply putting limits in place to prevent unintended consequences. Ethics would be stating under what circumstances a robot may purposefully strike a human in the first place.
"Robot, help me, I am falling into an abyss, give me your hand!"
"Sorry, human, but using a force equivalent to your weight would be unethical. I am going to report you to the HR department for trying to convince me to act against my ethical guidelines."
"The ISO’s update will, for instance, include guidance on the maximum force with which a robot may strike a human it is working with."
http://www.technologyreview.com/news/531356/should-industrial-robots-be-able-to-hurt-their-human-coworkers/