The Native app is free and there is an IN-app purchase for $39.99. There is also a full $39.99 version available as well.
There's nothing wrong with this business model by the way. It just want to know upfront what I'm getting (or not getting).
The paid version lets you use your own Linux/Windows ISO and/or import a VMWare/Parallels/VirtualBox VM.
This is basically how they can offer a "free trial" on the app store.