I don't see what the big deal is.
If Chalmers wants to run, let him... even if he does get defeated by Albo.
It's healthy to have competition.
I don't see why the opponents need to be told to drop out before the vote... otherwise why even bother with the vote.
Also, as a leader you are stupid to exclude quality from your front bench because they are your best competition... unless you are an Abbott who us just full on destructive.
Possibly 2 separate reasons. The first is that by suggesting that you might run, if you were any chance to garner decent support you can negotiate a higher level ministry be then withdrawing. Second is that the parliamentary wing don't trust or want the rank and file to have a say. If they come up with a different leader than what the numbers in the parliamentary wing want, then they will find themselves in a bit of a pickle.