My guess is that the real message behind the marketing-speak is that shapeways found
it too much hassle to get the vivid and even dyeing of finely detailed models right.
The tumble grinder probably has the additional advantage of removing the last few bits of
loose material that survived the normal cleaning. Based on Murphy's principle, I would expect
those to normally come off after dyeing, leaving white specks.
With black dye, one can probably just chuck such a part back into the soup - it can't get
blacker than black anyway. (Same goes for models where recessed parts got less than
their share of the dye.) With red and blue, repeated dyeing would probably make them too
dark, or at least come out uneven. I wonder how such problematic cases are handled now -
hope the customer accepts it as is, hand-paint over the spots, or even reprint behind the scenes ?