Because all of them are Americans and deserve protection under our common laws. All states are in the union with the understanding that the union will protect everyone, including unionists in a state with significant support for secession. It's not my idea of justice, it's our idea of justice, all of ours. Turning our back on them by letting a state leave illegally would be betraying that trust.

Also consider that before the U.S. civil war the southern states voted against Lincoln, the northern candidate, in fact he was not even on the ballot in many of them. If the enslaved people in those states had been able to vote things would have gone differently. So we have a unique and terrible history of states seceding based on a minority's wishes.