If the motivation of the Union was to destroy slavery which had been legal in the Union for “four score and seven years”, then why didn’t they attack Delaware, Maryland, Kentucky, or Missouri? Those were Union slave states, some of which kept slavery for six months longer than did the states of the Confederacy.
How can a Union which claims to be fighting against slavery (which they had no trouble keeping legal when they were in charge) be tolerant of slavery in their own states?
You will find the answer to your question in the Supreme Court Decision Scott V. Sanford.
Weren't you just demanding a Supreme Court decision to prove my point, and you are now offering a Supreme Court decision to prove the opposite of your point?
Are you now agreeing with Scott V Sanford? If not, this is why I don't suggest you put all that much faith into the pronouncements of a "Supreme Court".
Beyond that, this does not even slightly address the point. You cannot use a Supreme Court decision justifying slavery even in Free States, to explain why a Union Army which you claim was fighting to eliminate slavery, did not make the slightest effort to stop it in areas over which they had complete control.
Your response does not even make a bit of sense.