Apple wants developers to take 1/8th of your phone
gizmodo.com
gizmodo.com
There are already apps in the store with banner ads. It's one choice of many for a business model for the app. The purpose of the iAd platform is to make the user experience better for applications which are ad supported.
There's already nothing preventing paid apps from running ads (CNN Mobile is one example) and the iAd framework makes it even more tempting for app authors to do so.