Actually, Lee, that is quite a good explanation. I would add that the term "contraband" was a way for Union General Butler (who first used the term for formerly enslaved persons) to avoid having to return escaped slaves to the south, which would have been mandatory under the law of the time. Remember, this was before the issuance of Lincoln's Emancipation Proclamation.

By declaring that the escaped slaves who reached his lines were "contraband of war" he could justify keeping them, just as his army kept captured mules, stores of food, weapons and other useful things that fell into thier hands, thus denying their use to the enemy.

It's true that it seems cruel and racist to put human beings in the same category as livestock and stores of food, but Butler was a very shrewd person and effectively turned the slaveholders' concept of people as chattel against them. By declaring escaped slaves "contraband of war," he assured their freedom.