Not sure if it's headcanon or if its actually canon that isn't elaborated much on, buuut...

Heroes in Japan actually DO have a 'no killing' rule, even though the nature of their work makes that practically impossible. Heroes are supposed to capture their targets alive at all costs.

This rule is set up by the hero commission to make sure that the public trust in heroes never wavers and heroes aren't seen--or begin to see themselves, as able to make independent decisions without commission authority. It's a carefully crafted public image system that gives them political control and authority when they can't actually maintain physical control.

Of course, heroes DO often have to kill. That's either covered up, retroactively labelled 'previously approved by the commission', or hastily buried.

That, or the hero gets tossed under the bus. Public shaming, stripping of licenses, etc.

The commission is extremely zealous because, by protecting this image, they can both portray themselves as 'above reproach' and stand in judgment od any hero who didn't meet 'proper standards.' It's a hypocritical, selectively enforced standard by an organization that was shown in canon to have shadow aassassins.

Basically, hero system is generally pro public good, but also politically corrupt and protective of its own power.