First, a little history on MariaDB. MariaDB is a community developed fork of the MySQL. Essentially, when Oracle purchased MySQL from Sun, some of the developers from MySQL were concerned that the acquisition would make changes or lead down the road where it would no longer be open source.
So, they went ahead and forked off to MariaDB with the intent to maintain high compatibility with MySQL. Also, the contributors are required to share their copyright with MariaDB foundation rights, which in a nutshell means they want this foundation to always be open source.
This joins SQL Server, MySQL, and Postgres as PaaS database offerings in Azure.