The existence of robots is not problematic. Treating a human as a robot most definitely is. The existence of slaves, in the sense of things entirely controlled by other things, is not a problem, but treating a human as a slave is a problem (outside of consensual BDSM practices[1], I guess).
Words change in meaning constantly. Whilst I can see the appeal of linguistic conservatism, the facts of life in this matter really are not conservative. The use of "slave" to refer to devices that are controlled by other devices is only a little over a century old, so it's still quite a new coinage, but words can often change meaning on even shorter timescales. Quite a number of words in common usage today derive from words with different meanings and, over time, these origins tend to either be forgotten or come to be disassociated from the derived word such that we no longer perceive them as the same.
[1] http://en.wikipedia.org/wiki/Dominance_and_submission_(BDSM)...