So the whole "if they turn their back to you, your hands are tied" schtick? No. Doesn't work that way. Ethically, once someone violates your body through force, you are ethically excused from liability for responding in kind.
I'm not going to delve to deeply into it, but this is a philosophical discussion. You have the ethical right to defend yourself against an attacker, even if they hold their hands up immediately after attacking you. The police or other "legal" institutions in place that claim dominion over the provision of security don't change that.
I agree and the reason for this is that it prevents future violence and is not just vengeance. If someone hits you and then sticks their hands up so you don't do anything then they got away with it and will likely do things like this again.