Is it possible that Farook had some copyrighted material on his phone?

I think that's irrelevant. The encryption needs to be a means of control like DRM, not just copyrighted material that happens to be encrypted.

Edit:

>a technological measure "effectively controls access to a work" if the measure, in the ordinary course of its operation, requires the application of information, or a process or a treatment, with the authority of the copyright owner, to gain access to the work.

So encryption that doesn't depend on the authority of a copyright owner isn't affected.