I don't think the game theoretic approach is wrong, but you're ignoring that threat of censure (explicit or implicit, formal or informal) is an important part of the system, and in that dimension the informal ideals and "shoulds" are highly relevant, and need to be factored into the game theoretic analysis. If a prosecutor knows that they'll get accolades for exploiting a new way to screw defendants, that's a different system than if they know they'll face condemnation for it, even if there's nothing formal.
