Notability, which has some similarities, sells for 0.99 without in app purchase nonsense and is rated higher.
By making features an in-app purchase, they effectively monetize users going from version 1 to version 2, and so on. If you used the up-front payment model, new versions of the app would have to be a different app to continually monetize it, which would have a number of it's own problems.
I'm tired of paying for apps up-front only to find that they weren't what I was looking for or what was advertised. Good reviews don't necessarily mean that it is the right tool for the task, and forcing the user to then have to request a refund instead of just having a trial period or a free version they can simply delete is not user-friendly and results in all of the "DO NOT PAY FOR THIS APP!!!!1" and "I WANT A REFUND!" reviews from people who know none-the-wiser (although Apple doesn't make it very clear/easy to do so to begin with) and cause more people to turn away before giving it a shot. It also allows for the developer to be able to persuade initially unsatisfied users with later [demo] releases.
I would rather see Apple implement a time limited refund like Google does than use in app purchases for try and buy functionality.