1) People think slaveholders "took care" of their slaves out of the goodness of their hearts, rather than out of economic interest
There is a surprisingly prevalent belief out there that slaves' rations and housing were bestowed upon them out of the master's goodwill, rather than handed down as a necessity for their continued labor — and their master's continued profit.
This view was expressed to me often, usually by people asking if the family was "kind" or "benevolent" to their slaves, but at no point was it better encapsulated than by a youngish mom taking the house tour with her 6-year-old daughter a couple of years ago. I had been showing them the inventory to the building, which sets a value on all the high-ticket items in the home, including silver, books, horses, and, of course, actual human people. (Remember that the technical definition of a slave is not just an unpaid worker, but a person considered property.)