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 (please note that we're not related to F-Droid and don't have any influence on it).
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 consider donating to DAVdroid or purchasing it.