Pleasant Politics

It's not the same thing. The federal government probably can shut down any dispensary they want, but has made no effort to do so, largely because their actual authority to prohibit substances that don't cross state lines is not supported all that well constitutionally. That would likely end up in a lengthy legal battle either way. But it likely wouldn't require state agencies to actively enforce federal law, because there's no legal basis to force proactive law enforcement like that. (They can force action in other scenarios, eg forcing states to issue marriage certificates to gay couples, but the drug laws we're talking about don't dictate anything like that, and again, the actual authority behind regulating substances behind state lines is questionable at best.)

The fact that interstate commerce is explicitly entirely federal authority and that the Constitution explicitly prevents states from restricting it most ways makes it so there are no meaningful legal questions to an action like taxing exports. That ruling would be immediate, and anyone attempting to enforce it would be doing so in direct violation of a federal court order. There's plenty of legal basis to prohibit an action by a state that violates the rights of others.

And the federal government would absolutely make it a priority because states crippling interstate commerce would destroy the economy.