[ddt]

While the name "Uncle Tom" comes from the book, the character of Uncle Tom from ~1865 onwards was portrayed almost exclusively in minstrel show retellings of the novel. Minstrel show's didn't quite... capture the anti-slavery sentiments of the book [1].

The novel's Uncle Tom was resistant to the harsher institutions of slavery, sometimes standing in vocal opposition to his masters. The minstrel show Uncle Tom was almost exclusively played by white men in black face, going for cheap laughs by exaggerating the perceived mannerisms of American blacks. Essentially, the novel was radical, progressive and extremely popular. In the process of turning it into a minstrel show, everything radical and progressive was stripped out and replaced with cheap, comfortable laughs for an audience with a concept of how black people are "supposed" to act.

And that's how people who have just read Uncle Tom's Cabin don't get why Uncle Tom is now an epithet for people perceived to be subservient, or cooperating with their oppressors.

[1]: http://en.wikipedia.org/wiki/Uncle_Tom#Epithet