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)...
More immediately, it's Czech for 'worker': Karel Čapek, the person who coined the word as it's now used in English for his play R.U.R. (Rossum's Universal Robots), was Czech.
Karel Čapek: http://en.wikipedia.org/wiki/Karel_%C4%8Capek
R.U.R.: http://en.wikipedia.org/wiki/R.U.R.
These words are still in use anyway: master of your pet, master of a technology, a master class is nothing to be ashamed of... Slave/slavery is often used to describe what we called work 100 years ago. Plus some other connotations...
So why stop using slave and master as we use them in software? As long as they represent an abstract concept, they're harmless.