DAVdroid is designed to be an open-source project (GPLv3) from the very first beginning. It is always possible to compile the app yourself and use it for free. The source code is available, and you can download the app on F-Droid.
However, it was and still is much work to create and maintain DAVdroid, so we have decided to put it in commercial stores for a small fee. If you want to support this project, please donate to DAVdroid or purchase it.