It's part of the bullshit narrative they've been trying to sell for fucking YEARS. That the Confederacy rose up on an issue of states rights and not slavery. Then they conveniently forget what right the Confederate states were fighting to keep... spoilers, it was slavery.

Edit: and in case you are thinking this is just a Republican narrative, it's not. Democrats have repeated this bullshit too. Hell, the fucking West Wing actually had an episode where they repeated that bullshit, and had the audacity to have a black actress actually deliver it.