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.