It was no accident. Apple left even more images unencrypted in 10.0b2 (e.g. all ramdisks and 32-bit bootloaders!)
With beta one the company left the 64-bit kernel and the main file system unencrypted which was initially thought to be a bold move or a huge oversight. Apple later confirmed it was intentional.
In beta two Apple has left all the ramdisks (except ATV), 32-bit bootloaders, all kernels, and the main filesystem unencrypted.
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