Afaik Dec was pretty mismanaged at this time and Cutler didn't get his project greenlit. So "hired out" sounds more evil than it actually was. Microsoft saw the potential of Cutler's vision while Dec was busy being stupid. Why stay at Dec and work in something you don't believe in?
Oh, I didn't see "hired out" as evil. Whether it was mismanagement on Digital side to not greenlit Cutler's vision I'm not sure, but there could have been better ways even if they didn't want to replace VMS at the time.