Protecting the open-source license commons

I'd be very surprised if that was true, since I've worked with a few kernels for various SoCs and don't remember seeing anything in the kernel that wasn't built from source. The only exception I remember is that VTune supplied some profiling drivers as .ko files, but they were only used by a few people during development.

They do usually have kernel drivers that are designed to work exclusively with userspace binary blobs that the phone vendor doesn't have full source access to (for 3D graphics, cameras, etc), or to work with firmware blobs, but that's probably not a GPL issue since the kernel driver source is released and the binary parts are clearly separate from the kernel.