Regarding Prigg vs. Pennsylvania:
Writing for the Court, Justice Story reversed the conviction and held the Pennsylvania law unconstitutional as a denial of both the right of slaveholders to recover their slaves under Article IV and the Federal Fugitive Slave Law of 1793, which trumped the state law per the Supremacy Clause. Six justices wrote separate opinions.
Though Story ruled the Pennsylvania laws unconstitutional, his opinion left the door open for further such actions by the state in his writing:
As to the authority so conferred upon state magistrates [to deal with runaway slaves], while a difference of opinion has existed, and may exist still on the point, in different states, whether state magistrates are bound to act under it; none is entertained by this Court that state magistrates may, if they choose, exercise that authority, unless prohibited by state legislation — Justice Story (emphasis added)
This last phrase—”unless prohibited by state legislation”—became the impetus for a number of personal liberties laws enacted by Pennsylvania and the other Northern states. These laws did as the Court had suggested; they prohibited state officials from interfering with runaway slaves in any capacity. Runaways could not be caught or incarcerated, cases could not be heard, and no assistance could be offered to those wishing to recapture slaves. The Fugitive Slave Act still stood, but only federal agents could enforce it.
The Prigg case is particularly sad as 4 men pretending to act as slave catchers, assaulted and kidnapped a woman who had been given her freedom, and her two children, one of which had been born free in Pennsylvania. Prigg was properly accused of kidnapping, and his defense was kidnapping women and children was permitted by agents of the slave power.
The clause [rb: the fugitive slave clause of the Constitution] manifestly contemplates the existence of a positive, unqualified right on the part of the owner of the slave, which no state law or regulation can in any way qualify, regulate, control or restrain. The slave is not to be discharged from service or labor, in consequence of any state law or regulation.
Now certainly, without indulging in any nicety of criticism upon words, it may be fairly said, that any state law or state regulation which interrupts, limits, delays, or postpones the right of the owner of the slave to the immediate possession of the slave, and the immediate command of his service and labor, operates, pro tanto, a discharge of the slave therefrom.
I suspect some of the state laws resulted in blocking or greatly delaying the "immediate" return to service of the slave. As the Michigan legislature said, their law was intended to block the return of fugitive slaves.