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.