I think the reason slavery in the US was (relatively) humane (this in comparison with the Caribbean or Brazil, which the former IIRC was Hell on Earth) was b/c the slaves could not easily be replaced by imports.
Not to justify chattel slavery, but that's true to some degree. It is true that African slaves had a much higher survival rate than European "indentured servants", simply because they were worth 4-5 times as much.
Here's a little thing I wrote a while ago in another thread that's relevant to your question:
In most cases in the early colonies, there was no distinction between European and African slaves. The practice of chattel slavery originated in Barbados and the other Caribbean colonies, where the mostly Irish slaves were replaced with Africans, who were believed to be better able to withstand the conditions. Since the expense of enslaving and importing Africans was higher than that of importing Europeans, Africans became enslaved in perpetuity (the visible racial distinction between Africans and their masters was also a factor). The brutality of how indentured servants and Irish slaves were treated was very often worse than the treatment of enslaved Africans, since there was no economic incentive to let a European survive the terms of his indenture, and rarely any legal incentive to treat slaves of any color well at all. African slaves cost four to five times as much as European bondsmen, since the Europeans' children would not also be slaves, and whites were not considered an investment. The practice of chattel slavery was brought to Charleston from Barbados, where it went in hand with the whole plantation economic system.
Meanwhile, in the other colonies, the offspring of various enslaved or indentured people (Africans, English, native Americans, Germans, Scots-Irish, Irish, and the Iberians who were the ancestors of the Melungeons), as well as those whose indentures had expired, drifted to the hills and intermingled. Their descendents are most of the peoples of the American interior, the "hill-billys", as well as more conspicuous groups like the Melungeons and Lumbees. The 'one-drop' color line, as part of the plantation system, originated in South Carolina. In Louisiana and Latin America, people of mixed descent could rise to the highest parts of the social order, and intermarriage was quite common in New York and New England prior to the Revolution. The political influence wielded by the Southern planters after the revolution (among many other factors) managed to split Americans into the categories of 'black', 'white', and 'Indian', into which huge numbers of Americans could not comfortably be fit, and setting the stages for the ensuing tragedies of American history.
Of course, I fail to see how the "relative humanity" of American slaveowners has anything to do with the current argument about the Confederate cause.