Android as a whole is, indeed, a derived work of the Linux kernel. But that doesn't mean that all of the component parts of Android are derived works of the Linux kernel. You can, and companies do, make changes to the BSD part of Android and ship them, without making the code publicly available.
Here is another example. If you're a book seller, you can put several books on the shelf-- even ones with the same subject-- that have different copyrights. You do not need to get permission from the authors to do so. Just putting two things side by side does not make one a derived work of the other.
And we haven't even talked about "fair use"-- the quantum physics of copyright law. What a confusing topic!
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