We pass a law saying: "You cannot have P."

The market incentive for producing P is now vastly reduced, because the black market is a fraction of the size of the real market, so P never gets built in the first place because it takes a lot of investment and expense to do so.

More nations have nukes than they used to, but far fewer than have guns. Life is lived in a gray area that the "not everyone will follow the law therefore the law is useless" mindset somehow ignores.

And if "P" is "an AI that controls weapons deployed/used in the US," say, then enforcement is likely to be more effective than just "nobody can have AI at all."

So it's probably worth thinking about what specific things we do and don't want AI to be used for, and which of those are more or less practical.

EDIT: oh, and it also assumes that P is something people want for itself, vs "if they're gonna do it I better do it too" or "it's 10% cheaper and the externalities don't directly effect me THAT much" or other similar behaviors that can emerge without regulation. Banning leaded paint and leaded gas vs banning alcohol, say.