This is funny, because I have previously made the exact same argument regarding Article IV, section 2 of the US Constitution. Unless you prohibit the right to transit (violation of the privileges and immunities clause) you can't keep slavery out of "free" states.
You get the argument now but only because you can see how it works in the Confederacy. I suspect you will still be unable to see that it works exactly the same way in the Union.
It's hilarious actually because unlike the Confederate Constitution (Article IV, Section 2) nothing in the U.S. Constitution protects the right of the slaveholder to take his/her slaves to any state in the Union. Also unlike the Confederate Constitution (Article I, Section 9), there is nothing in the U.S. Constitution that prohibits laws impairing the right of property in slaves. Finally unlike the Confederate Constitution (Article IV, Section 3) nothing in the U.S. Constitution prohibits Congress from banning slavery in the territories. So According to the 10th Amendment, laws about allowing slaves in a particular state would be up to the state itself. And according to Article IV, Section 3 allowing slaves in the territories would be up to Congress. Your premise is wrong.
You get the argument now but only because you can see how it works in the Confederacy. I suspect you will still be unable to see that it works exactly the same way in the Union.
Looks like you suspect wrong.