The Cato institute has been doing its best to repudiate libertarian ideas lately. The idea that secession was in essence "kidnapping" is silly. In order to believe that theory you have to assume that the slaves were somehow being taken away from the Union. The Union didn't want the slaves and Lincoln was not interested in freeing the slaves. He was attempting to keep the union together for economic reasons.
The South was trying to take slaves out of US jursidiction, which probably would have meant prolonging their bondage. The Civil war was not primarily about ending slavery but this was nonetheless a, to some extent unstated, secondary objective without which far fewer northerners would have been willing to fight.
By comparison in the War of Independance slavery was a very minor issue. Britain may have had it's abolition society by 1774 but it was hardly government policy by 1776. Had the revolution been delayed until 1807 then slavery would have been a major issue & the balance of right would have shifted to the British side. Equally had it been delayed that long it is unlikely that the northeren colonies would, by then, have supported an independance joined to slaveholding states.