When you say "KC", do you mean the City of Kansas City, or do you mean AEG, the managing entity for the arena?
Kansas City will get money from AEG (I assume) no matter what the tenants are, based on the managing agreement -- at least if it was written well, which I realize might be an overly aggressive assumption.
Now, if you mean AEG, what AEG would get would be an instant cash infusion from the Pens (buying that management stake, not getting it for free). Also, they need to look at the money side of what they'd get WITHOUT the Pens coming to KC.
Without the Pens, AEG gets 100% of an arena without a primary tenant.
With the Pens, AEG gets 50% (under the proposed arrangement) of an arena with an NHL team with 41 regular season games (plus pre- and possible post-season games)... which MAY be projected by AEG to be more money than the 100% of a much less-booked arena.
THAT might be why they would make this offer.