I've seen remote execution backdoors like this in a compromised Joomla site before (at least w/r/t the code snippet that actually does the execution), so I would not be in the least surprised if someone was clever enough to use the copyright file as a place to sneak it in.

I saw one attacker drop the same backdoor php file in a dozen different locations, in hopes that the site owner would just remove the one in particular that was showing up in their logs and none of the others.