Why not? As long as a DNF is guaranteed, they're not competing against anyone (I'm thinking more of track than road events here) and it might help them do their jobs better. This isn't to say that they're not already jacked, since they're presumably not tested, at least not by anyone invested in catching them.
Think of the interesting situation that would arise if doping were sanctioned only for athletes who agree to serve only as pace setters, then along comes a Keino type with a hematocrit of 55% who takes a 5K field through 3K in 7:30 but feels so damned great he refuses to quit and runs away to "victory" in 12:35, with the literal "winner" ten seconds back. It's the fastest 5K ever and by definition is a sham, so can it still be admired?