For many people arguing his side, facts tend to be malleable depending on what they need for their argument of the moment.
Everyone seizes on that phrase "all men are created equal" in the Declaration of independence to prove that all those slave holders who wrote and signed it intended to abolish slavery. That they continued to hold slaves is dismissed or excused away because it simply contradicts the narrative they want to push; That the war was fought by the Union to abolish slavery rather than for the ignoble cause to oppress and subjugate a people who wanted independence.
As the 1860 crowd well knew slavery was a very common institution and the vast majority of Presidents before Lincoln had been slave owners at some time in their life. The notable exceptions that I know of being the Adams family and Franklin Pierce.
Many people have a difficult time comprehending the zeitgeist of that era. Many don't even try. They simply apply their anachronistic modern notions so they can feel smugly superior to people who grew up with a different world view.
I think what happens is that the reasons for secession and the war get conflated.
Secession occurred over slavery, over the Deep South deciding that they needed to separate from the increasingly belligerent abolitionist faction in the North. John Brown's aborted slave uprising and the widespread support he received in the North was the most blatant example of a decades long campaign of hatred against the South. Thomas Fleming wrote a good book on that subject A Disease in the Public Mind
And while Lincoln wanted slavery to end, his announced reason for going to war was to preserve the Union, something which he reiterated many times. But in the modern mind the Civil War becomes a war launched with the intended goal of ending slavery. Now this is an interesting thought, because they in effect are saying that Lincoln decided to engage in slaughter and destruction in order to further a political goal, something Obama hasn't quite yet managed to try, although there's still time. And with respect to Lincoln and Obama I've been trying to figure out if the emancipation proclamation is any different from an executive order. But I digress.
In resorting to war Lincoln was doing something more in line with Buchanan's long forgotten Utah War and Andrew Jackson's threatened invasion of South Carolina over nullification. Both of those were exercises in the national government using military force against territorial and state governments to compel compliance with already existing law, they weren't attempts at using military force as the means to change already existing law, which is what a war started to end slavery would have been.