Commision on exchange games.

Does anyone know why betfair charge commision on exchange games. It isnt like their sports betting where you are betting against other members, you are betting only against Betfair, so on the odd occasion you manage a small win against them why should they take a bit of it back ?? After all they will already be quids in from the losers on the same game.
Also ive heard it said that the cards are drawn in an order that will make sure that the hand that wins is the hand that gives Betfair the most profit and are therefore pre-determined and not randomly generated. This cant be true can it  (GULP !!!!)