Man that spielberg is great. I hope he can find another really obvious cause. Or he could address that the bullets made in Schindler's factory fired just fine, but that would be adding shades of meaning, a task of which he's obviously incapable. "If it isn't black and white, I don't want to see the script on my desk."
As for "how could they do that?" it's a really good question. I'm familiar with Jefferson and Washington's take on it. Washington of course freed his slaves, and Jefferson didn't because he was broke by the time of his death.
Jefferson petitioned for ending slavery, and he almost succeeded. He ultimately failed because South Carolina wouldn't have stayed in the Revolution/obeyed the Constitution had slavery been ended.
If you want, a good book is _The Wolf by the Ears_. Also look for Jefferson's notes on the state of virginia re: slavery. Racist: yes. Pro-slavery: no. This was the case with almost every white in America until, well, recently. Even Lincoln.
Before anybody responds, Jefferson probably didn't make babies with Sally Hemmings. A Jefferson--there were over twenty in three counties--did that, and Jefferson was 65 when Hemmings' child was born.