Good question. Lincoln was very keen to dampen, not fan, the flames of war during the early years of the Civil War for a couple of reasons. First, border states like Kentucky were still on the fence about whether or not they were going to secede. Lincoln didn't want to give them a push one way or another by acting like the rabid abolitionist the Southern (and Democratic) press made him out to be. Until the Emancipation Proclamation, Lincoln's policy on slavery was very tame compared to what he endorsed after 1862. If Lincoln's administration confiscated slaves and "freed" them by way of not returning them to their masters, Lincoln threatened this tenuous neutrality in the border states. Second, early in the war, before anyone really understood how protracted a conflict it would be, Lincoln was keen to not antagonize the Southern people any more than he had to. Lincoln believed (hoped?) that the war would be over within a year or so, and didn't want to fan the flames of discontent that had, in-part at least, started the war. Thus, the policy in the early days of the war was for Northern (Union) troops to return slaves to their southern masters if a dispute came up. Gen. Benjamin Butler changed all of this when he (all on his own) decided that slaves were contraband of war, and therefore liable for confiscation by the Union forces. Butler was quietly scolded for taking such a sensitive issue head-on without first getting approval from the White House, but his policy largely stood (on an informal basis, anyway) until 1862, when Butler's contraband policy became the de facto policy of the Union forces. The reasoning behind it was that slaves were contributing to the Confederate war effort, and therefore were liable for confiscation by the Union.