Right, and neither do Google's processed header files only produce definitions that cover syscall related constants and structs. Just because they excise everything guarded by __KERNEL__ doesn't mean they aren't capturing copyrightable code; similarly, just because something is guarded by __KERNEL__ doesn't mean it's copyrightable. The law says what's copyrightable, not the author; the author can only license--explicitly or impliedly--what he has copyright in.
Go through those files yourself, and then ask yourself if you would mind if such collections of your own code of similar complexity and expressiveness to be entirely without copyright protection.
The point is that _some_ of the output of the processing may be copyrightable; *not* that _all_ of it is. There's significantly more there than the syscall descriptor number for the read system call.
So you have to ask yourself, is the convenience to Google worth the circumscription of copyright protection for copyleft works? Perhaps it is; but don't pretend that their legal theory doesn't have repercussions.