For what reasons does Augustine St. Clare believe it is inevitable that slaves grow up to be dishonest?

From the text:

"Well, now and then one, whom Nature makes so impracticably simple, truthful and faithful, that the worst possible influence can't destroy it. But, you see, from the mother's breast the colored child feels and sees that there are none but underhand ways open to it. It can get along no other way with its parents, its mistress, its young master and missie play-fellows. Cunning and deception become necessary, inevitable habits. It isn't fair to expect anything else of him. He ought not to be punished for it. As to honesty, the slave is kept in that dependent, semi-childish state, that there is no making him realize the rights of property, or feel that his master's goods are not his own, if he can get them. For my part, I don't see how they can be honest. Such a fellow as Tom, here, is, -- is a moral miracle!"