It would be patently ridiculous to expect that anybody would willing to fight to ensure the continuance of their own bondage. Of course they switched sides - anybody in their position would. As to why they first offered their services to the Confederacy I would offer what psychologists call 'Stockholm Syndrome' wherein captives come to identify with their captors and cooperate with them. On a small scale we saw this with Patti Hearst and on a larger scale with Jews who served as Capos in the death camps of Nazi Germany.
"An acknowledgement of the errors of the past is not an assumption responsibility."
"Extraordinary claims require extraordinary evidence."