The Sox knew they were giving up a premium prospect. At the same time, I think Dombrowski - not unreasonably - decided that the high variability that comes with an 18 year old pitcher in Single A was enough reason to cash him in to help the major league club who had a division to win.
I did not love the move - but my disagreement is more in the "I disagree, but I get where he's coming from, and his view is pretty reasonable" vs "What the heck is that guy doing?"