1. It is well established in US copyright law that elements necessary to interoperate with something aren't copyrightable. So, for instance, the function signatures and probably function names (if names are necessary in order to make the call) of an interface would not be copyrightable. Same for structures used in the interface.
This has been fairly extensively litigated. It arises when you have, for instance, someone trying to make an unauthorized third party game for a video game system, or someone trying to write a plug-in for someone else's closed program.
2. Header files often contain more than just the bare elements necessary to interface to something. They often contain macros that expand to code, comments, maybe even implementations of small inline functions. There is plenty in header files that can be copyrighted.
So, depending exactly on how Google produced its header files, they may be in the clear or they may have made a big mistake.