This is a pretty loaded question.

First off, remember that most Union soldiers came from states that had abolished slavery, and many came from states like Massachusetts, which was a center of abolitionist sentiment. So saying that all (or even most) soldiers continued to oppress black people isn't necessarily accurate.

More importantly, the slavery / race issue wasn't the most important motivation for a Union soldier. More compelling was the idea that they were fighting to save the Union, that the southern states were in rebellion and needed to be brought back. There were also, of course, financial inducements - many local and state governments offered bounties for new recruits.

Of course, many Democrats (especially the later Copperhead faction) tried to discredit the government by claiming that the war was being fought for the rights of black people. This had some effect, but the success of Union forces blunted it, especially after the fall of Atlanta in 1864.

Wiley's "Life of Billy Yank" spends a good deal of time on the questions of why the average Union soldier fought and their views on race. His conclusion from reading soldiers' letters was that the majority of them were fighting to preserve the Union. Most of them shared the white supremacist views that were usual at the time and many of them were unhappy after the Emancipation Proclamation, since they perceived this as shifting the purpose of the war to abolition and they didn't necessarily want to die for black people's freedom.