To answer this, think of why does Facebook allow users to purchase ads from within the app? Does it make for a better experience to be able to buy an add from your couch, or while traveling? Isn't the Facebook experienced for these users improved by offering for sale goods within the app itself?

I think there is a very good case to be made that the in-app experience is improved by allowing these purchases in-app, and the in-app experience and value would be significantly reduced if Facebook did not allow marketers to buy ads using their iPhone or iPad.