Whether or not it has _GPL is completely irrelevant.
The question is "does my modification to the kernel make a derivative work?" The _GPL defines are there to warn developers "Yes, using this function means you're probably doing something derivative."
In short, Oracle knew what they were doing, knew it was wrong, and are doing it anyways. Now it just comes down to the lawyers arguing whether DTrace becomes a derivative work or not, when and if someone files suit against them for not being able to use DTrace on TheirLinux.
The question is "does my modification to the kernel make a derivative work?" The _GPL defines are there to warn developers "Yes, using this function means you're probably doing something derivative."
In short, Oracle knew what they were doing, knew it was wrong, and are doing it anyways. Now it just comes down to the lawyers arguing whether DTrace becomes a derivative work or not, when and if someone files suit against them for not being able to use DTrace on TheirLinux.