It barely needs to be more than a line in the ToS for the API. I'm sure Twitter already had a list of terms, and a way to ban people who misbehave. This is one more way to misbehave and get banned. Going right to an App Store would be insanity.
Showing ads isn’t enough. You need to count as impressions to bill advertisers and convince them to trust the impression data coming from third party clients.
