There's no external reason to actually follow the ethical rules, as long as you create the appearance of doing so.

The internal reason is, when you fail to follow the rules, you have to continue the facade. Is it easier to perpetuate a facade, or is it easier to just follow the ethical policies in the first place? (Some people do choose the former, and maintain the facade for a long time, if not forever -- for every preacher caught with his pants down [literally], there's probably a dozen who don't get caught.)