The southerners wanted the right to re-enslave free blacks in the northern states. Further the Dred Scott decision was an example of a black man who was taken to free territory by his master, and after the master’s death his heirs wanted the institution of slavery legal in a southern state, but not in Illinois to be enforced against him.
Illinois had the state right to forbid slavery. When a slave owner violates that state’s laws, he can not then claim the law must return the slave to a state where slavery was legal, as if the owner’s illegal action had no consequence.
Which Southern state petitioned for re-enslavement of free blacks?