So people could download and try it for free, which is what we do for the direct version as well. A lot of people won't download an app if it isn't on the App Store, and then they won't be willing to shell out $30 just to see if it's what they want. With the free trial + IAP, we can cover both those cases.
Honestly, we thought Family sharing would have covered the IAP as well. I'm surprised it just isn't automatically done.