They also provide leg-ups by negotiating feature of their applications, etc.
I think not only do large companies get a much faster and simpler submission process, but I also believe the current landscape of the entire app store has an impact on your submission. If 1000 action games just got submitted and that part of the store is currently saturated, I am positive Apple will hold back any more action game submissions for a while and possibly even reject perfectly valid submissions.
After all, Apple is making money off these apps too, they have a vested interested in maximizing what goes into the store.
However, the bubbles used by the Facebook app are exactly the same as the one in SMS/iChat.
Maybe it passed through the review process by mistake, but you have to wonder…