It's sold through the Mac App Store only right now, so it's limited to what the Mac App Store allows, unfortunately.
Maybe in the future, if it goes well, the developer might consider selling outside the Mac App Store, and this have more flexibility. I have no knowledge of his plans on that matte,r though.
Been a while since I looked at App Store policies, but couldn't they charge a discounted price outside the store and then give a promo code to the app?
I believe that it's not intended to be used like that (promo codes are also limited to 100 per application version now).
Also, this would require the developer to find a way to charge people outside of the store. In that case, it's almost half way to setting up a full direct store with a direct version.
There is benefit to having a direct version (all our applications do) but many developers today don't want to set this all up. It's quite a lot of work and perhaps the developer is not interesting in spending time on that. Moreover, he might not even be sure if the app is successful enough to invest time into this work.
Maybe in the future, if it goes well, the developer might consider selling outside the Mac App Store, and this have more flexibility. I have no knowledge of his plans on that matte,r though.