[michaelt]

  Why is it considered acceptable to enslave 
  AI robots

AI is largely hypothetical at the moment, and whether such things are ever acceptable or not will depend on the form AI eventually takes, if it ever exists at all.

"The Restaurant at the End of the Universe" by Douglas Adams includes an animal that has been genetically engineered to want to be eaten [1]. If AI took a form where their personalities could be precisely configured, they could simply be configured to joyfully consent to whatever fate their creators had in store for them.

[1] https://www.youtube.com/watch?v=C1nxaQhsaaw