Fugitive slaves are one thing, on that everyone agreed.
But permanent residence of slave-holders with slaves in free states was never considered acceptable before the 1857 SCOTUS Dred-Scott decision.
So, there's nothing to "grasp", you've simply misinterpreted Founders' Original Intent.
And whether Roger Taney agreed with you is irrelevant, because he was a lunatic.
What has that got to do with what the law says? Sanctuary cities and medical marijuana is considered "acceptable", even though both are clear violations of federal law.
The fact that nobody was enforcing it led people to believe it was "acceptable" to ignore it, just as Liberal kooks are trying to do nowadays with illegal aliens and weed.
So, there's nothing to "grasp", you've simply misinterpreted Founders' Original Intent.
That is easy to do when no stretch of the words can comply with what you claim was their "intent."
Their "Intent" was to get the Southern states to sign on, and so they put in protections demanded by the Southern states, and they put them down in writing.