First up it's important to remember the context of the time period and these people's lives. This was done in the midst of the Great Depression so the people interviewed generally weren't doing very well. Second the people interviewed were almost always children or teenagers when they were freed meaning that they probably had not faced the absolute worst that slavery had to offer and they might even long for their childhood as many people do. Third they may not have wished to speak ill of the dead, even if they had been their former owner. Finally many of the people interviewed may not have understood exactly why they were being interviewed and may have thought what they said could get back around to them, in either a positive or negative way.