Without unleashing the market upon pot, criminals could still make large amounts of money on the black market. More to the point though - as the article says,

Without unleashing the market upon pot, criminals could still make large amounts of money on the black market. More to the point though - as the article says, the burden of proof is on the side advocating restriction! We don't require governments to allow us to do every little thing - the basic assumption is we can do anything we like, except for a few restrictions needed to prevent reducing others' freedoms.