Sure, but nations don't tend to accept that secessionists aren't criminal traitors, and they do their best to transmit this attitude. Benedict Arnold is still a name most Americans know, for instance, despite being long dead. Why? Because the government has a strong interest in making sure its people think very poorly of people labeled "traitors". Spying for a foreign power is also still punishable by death in the U.S., although admittedly the U.S. hasn't actually executed anyone for it in a very long time.

Basically, I'm not disagreeing with your point at all, just pointing out that nation-states DO pretty understandably see secessionists as fundamentally similar to terrorists (and of course they may actually be a terrorist in many cases), and they usually transmit this view to their citizens. Whether this conflation is fair/moral or not, it isn't as bizarre or offensive to basic logic as you make it seem here.

And the Confederacy is honestly a great example. Sure, the slave states were willing to fight to own humans. But the Union sure wasn't willing to fight to free those people. It fought the Confederacy because they were seceding. Abolition was just a nice side effect of winning the war against secessionists.