That's exactly what I'm saying: they were fighting to preserve the union, not for abolition of slavery. That's why the Emancipation Proclamation just muddied the political waters.
Yes it was about preserving the Union and also more than that. There was a lot of anger in the north at the south for seceding. Even more after they fired on and captured Fort Sumter. To the Unionist the southern states annulled an election that they lost fair and square. Many unionist saw this as a dastardly, dishonorable act. It reeked of unfairness and disrespect, to put it in modern language it was a big F*&^ you to the United States. Secession busted norms and broke associations that had been in effect for decades, and did so unilaterally.