Room Availability Calendar is licensed under the GNU General Public License v2 or later (GPL-2.0+). The full license text is available at gnu.org/licenses/old-licenses/gpl-2.0.html.
You're free to use, modify, and redistribute the code. Purchasing Pro gives you the current release, future updates, and support — not exclusive ownership of the code itself. That's how GPL software works, the same way WordPress and Joomla itself are licensed.
Questions about licensing? Contact us at info@darius-design.net.