She could probably throw her support behind him, but she doesn't need to feint so elaborately to do that. It will simply suck the oxygen out of the room for other conservatives to rally around her, not him.
I think the simplest explanation is the best. Palin looks like she's running because Palin is running.
And when she does, the field suddenly transforms into Palin vs. whomever will take take the role of "not Palin" (presumably Romney, possibly Pawlenty). With everyone else running for VP or the offhand chance that the leaders completely implode.