What do you think will happen to the salt...
Over millions and billions of years of adding fresh ice on the top of the stack, and just occasionally having a mild thaw - yes, it WILL melt, but it will then run away downhill, transporting the salt AWAY from the icecaps.
So, o0ver geologic time, the poles will tend to become salt-deficient, and melting will become improbable.
That's the way things work.
Now, if Phoenix lands and finds a HIGH salt concentration, then of course I'll eat my words, but every expectation is that the poles are a store of relatively CLEAN ice.