Often used when discussing Black conservatives, this term refers to the notion that white liberals prefer for all African-Americans to fit into one political viewpoint, generally characterized as left-Populist-Democrat. If Blacks all think the same, then the leadership of the liberal white left can regard Blacks as a monolithic voting block. That makes things a lot easier to strategize and manipulate than if the Black population is seen as a whole spectrum of political views -- which, of course, is indeed the case in reality. Thus, when white liberals imply that any politically-aware Black who is not left-Democrat is either suicidal or insane, they are "keeping the Blacks on the liberal plantation" -- and Blacks who are not left-Democrat are spoken of as having "bolted off the liberal plantation." An important facet of this paradigm is that the "liberal plantation" is figuratively a "plantation" because it uses Black voters to serve mostly the interests of a white liberal establishment, not the interests of the Black American voters themselves.
"The Black feminazis ganged up against Clarence Thomas during his Supreme Court nomination not because they thought he was sexist, but really because he had the nerve to bolt off the liberal plantation."
"Black Libertarian Thomas Sowell is sometimes called an Uncle Tom simply because he has wandered off the liberal plantation."
White conservative TV pundit: "Herman Cain is being attacked for sexual harassment by the liberal mainstream press not because he did anything substantially improper, but for bolting off the liberal plantation."