If they had gone with the traditional free lite version / paid premium version model, you would lose all of your drawings when you upgraded. This is just a free trial for the real product which can be unlocked at any time with minimal friction. Personally, I prefer this to having to hunt in the app store for the "premium" version if I decide to make the switch.

Doing this also gives the developers the option to introduce paid updates in the future, which is potentially good or bad.