you had to decide on a rule that both you and others had to follow.

I can't do that. I have to decide on a rule that both my and others follow and that can be effectively enforced. Things may well work better overall if no one spanked their kids, or they may not - just because bad parents overuse spanking does not mean preventing it will make them into better parents. But it would be really, really expensive to enforce a rule that prevents other people from spanking their own kids. It is not that difficult to prevent people from spanking other people's kids. "Difficulty of implementation" is an excellent reason to support one rule but not the other, in my view.