It's the way a lot of applications in the App Store work. Free with ads or pay this fee to remove the ads. With this, you are getting the App without ads, for free.
I mean honestly, how do you think Developers make money when they offer their app for free?
They should just offer their apps at a price then or a free demo version with half the features of the payed version. All the ads I've see are garbage, intrusive waste of space. I highly doubt anyone bothers to click them as they don't offer anything useful at all.
Lol, reviving a 3 year old thread.. Well you could always use AdBlockerNetworks, this tweak often works, but know that by using it you take away some well deserved revenue of the developers.