Not true. Zynga adopting FB credits for all of its games is very much a good thing for smaller developers on the FB platform. Having Zynga on board is a big step to having FB currency used as a standard for all FB games, which levels the playing field for small developers who can't build huge payment systems themselves.
Unless Zynga has 10% fees (my guess) and everybody else the usual 30%. I think they wouldn't sign the agreement otherwise.
There's no problem with using SocialGold, Spare Change (Playspan), Gwallet, TrialPay, Offerpal, SuperRewards or many other payment/offer systems that are easy to integrate into Facebook apps.