being forbidden by law means that it carries a punishment

No. Being forbidden by law means that the Powers That Be will take forceful steps to stop people doing it, even if they want to. At present, that generally means punishing people who do whatever-it-is, but in some hypothetical future surveillance-heavy society it might instead mean that as soon as someone tries to do whatever-it-is they are physically prevented by an armed robot police officer. Or in some hypothetical future brainwashing-heavy society it might mean that those who do whatever-it-is are tracked down and have their brains modified so that they won't (or can't) do it again. In yet another hypothetical society it might mean that anyone discovered to have done whatever-it-is feels so ashamed that they commit suicide.

It happens -- and this is PhilGoetz's point, I think -- that in (almost?) all existing societies, in (almost?) all cases, a central part of how legal forbidding works is via punishment of those who do what's forbidden, even though this isn't the only conceivable way it could work.