It's because you only get the trial version to show in cydia. You download the app, then activate it inside your device via the (for lack of a better term) in-app purchase method (ie. input your email, registration, code, whatever it asks for). It will then activate to the full version. A number of apps do this that have payment system set up outside of Cydia. I think (don't know for sure) that this is the only way currently that they can offer a trial version.