The official story was that they weren't Slavs at all but Germans who spoke a slavic dialect. Thus they were Aryans and were not abused (at least more than other Germans were). I believe the treatment of the Masuren in Ost Preussen was similar.
Strange of course; but then there were bizarre categorisations applied to Poles too - some were deemed "nordic" others "sub-nordic"and so on. Moreover; the different administrators of occupied Poland had different policies to their hapless subjects. Frank wanted to simply re-label Poles as Germans basically while others wanted eventual mass deportations to Siberia.
I suppose the lesson is that one cannot find much rationality in a system overtly based in the irrational (myths, Volk, Blood etc).