Thats definitely a new revenue model for the app. Starting with version 5.7, which arrived in late March, the app changed to a freemium structure. While the app itself is free to download, in-app purchases allow users to do things like remove ads and add push notifications.
Ive been playing around with the latest update for the past few hours, and its definitely a breath of fresh air in terms of Twitter clients on iOS 7. Twitterrific has always been focusing on actually reading your timeline, and this update hones that experience.