well, I'd guess in this case the chord is so named because whoever named it saw it as functioning as a vi chord in Eb. Thinking in that context, the chord name becomes "Cm (add your extensions here)"
So "Cm7sus" is really not a bad name, it's based on the harmonic function of the chord, rather than a literal spelling of the chord. The whole chord name thing is a kludge anyway, it's good to know as much as you can about the conventions of chord naming, but don't get too hung up on it. It was developed as shorthand for studio musicians, it's not supposed to be a rigorous system of harmonic analysis ;)