I'd like to think of it as one of the things that sets apart PR from your average sleazy reality show. They're not interested in watching everyone kill each other until there's one left standing. They are happy if any of their contesants make it in fashion. At reunion shows, they proudly announce if one of the losing designers ended up getting a Bryant Park show on their own, like Melan or Victorya, even if it seems a little strange that losers can obtain the same prize as the winners. The exposure on the show was what moved things along for these people, so it's not like it hurts the show.
So if having decoys is the easiest way to avoid spoilers for the show, they see it as a win-win. The decoys still get the expereince of a lifetime, and exposure beyond their wildest dreams, even if they have to wink a little for a few weeks and play dumb. Everybody's a winner in some way.