The real reason is twofold, first and more importantly, because you don't know the unintended consequences of proposing something like that. Who knows what else would get caught between the regulatory framework needed to prevent someone from doing math, because let's face it, that is impossible so unintended consequences will be the only consequences.
And secondly and most importantly, the government should not decide what products are allowed to be traded: Governments should lift all bans on products currently banned, all drugs, all books, all music, all banned clothing, etc.
The last sentence comes off as a rhetorical sleight of hand. You can be against censorship of books and music and still believe that the government has a role to play in regulating dangerous goods like plutonium. The trade of goods with an outsized environmental impact is regulated today, though this mostly shows up as restrictions on chemicals that are themselves direct pollutants.
Sure, you can be against anything, pro anything, and believe in anything. What I believe is what I wrote, you can believe governments should control plutonium if you want.
And secondly and most importantly, the government should not decide what products are allowed to be traded: Governments should lift all bans on products currently banned, all drugs, all books, all music, all banned clothing, etc.