There is nothing wrong with "copying" kernel. This is what the open source is aimed at: Infectious growth. In fact that was the right and smart thing to do.
'Copy': make a similar or identical version of; reproduce: [1] Google forked and contribute to the kernel. They did not create their own identical version of, or reproduce, the kernel.
[1] http://www.oxforddictionaries.com/definition/english/copy