They simply wanted to keep him on his regular pitching schedule through the ASB and that was the most convenient way to do so. He'll be called back up at the first possible opportunity for his next ML start (10 days after getting sent down). This will let him get in a start so he doesn't end up with a 10 day lay-off.
Hope springs eternal
grows like ivy on the vine
then dies in the fall
--Haiku for a Cubs Fan, The Loveable Losers