Why on earth would they invite non-candidates like Guiliani and Palin?
Ratings.
Only reason for anything and everything they broadcast.
They probably think that if Sarah Palin showed up, as either a spectator in the stands, or a participant (should she announce prior to that time), that a few people on this planet might tune in that otherwise would not.
Of course, not seeing any value in having Sarah on the show, I could see why some would question the producer's wisdom in thinking that.
Poll numbers.
Bachmann was a non-candidate who participated in the first debate.