MyBB Community Forums

Hello,
Sorry if this question is in the incorrect topic, I did not know where is the place of this question.
Why when I open any PHP file of the MyBB's source code in the top message says all rights reserved if MyBB's license is a GPL license? MyBB is not completely free software? What is the reason of this?
Thanks in advanced.

My understanding:

Because the GPL depends on the copyright-holders retaining their rights: if the copyright-holders gave away their rights, then the software would enter the public domain, and the Free / Open Source licence - in this case the GPL - would be unenforceable.