In other words, they knew the South had the right........but they couldn't afford to let them go, as a practical business matter.
Thought so.
Glad you brought that up. Let's just talk about practical business matters for a moment.
As a practical business matter, the South could not free its slaves. To free the slaves meant bankruptcy.
Instead, the South worked very hard to ensure that a balance between the number of slave and free states was maintained when new states joined the Union. This was driven by practical business considerations: they knew that eventually the free states would outnumber the slave states to the extent that slavery would be Constitutionally abolished.
It's worth repeating: left to their own devices, most new states would have been created as free states. It is rather telling that those worthy advocates of "states rights" worked so hard to ensure that new states could be forced to be slave states -- and that they considered slavery a good enough reason to leave the union.
One is hard-pressed to defend a group of people who are willing not only to maintain slavery, but to spread it. And with that in mind, the underlying reason for this particular act of secession is a rather damnable excuse for secession, whether or not it be legal.