If the federal government had absolute authority in the matter of slavery, why were the States even 'allowed' to choose?
Touche, excellent counter argument...
After the Dred Scott decision, the states pretty much lost control over any decision over slavery inside their borders. Scott said among other things,
1. The Northwest Ordnance was unconstitutional (complete historical nonsense)
2. that individual state's personal liberty laws were trumped by the Constitution's provision requiring return of runaways (probably constitutionally correct, but something you think "States Rights" fundamentalists would have a problem with.)
3. That blacks (free or slave) were not people as defined in the Constitution and had no rights in any court in the land (Federal or State).
The "intent" in Scott was to protect slavery in any federal territory. But reading Scott, there is nothing that would have prevented a slave owner from taking his slaves into a free state and setting up shop there.
As Lincoln said in the House Divided speech, we were destined to become all one thing, or all to other.