Or rather, why did rms et al dislike UNIX? It seems to come up a lot that they basically settled for a UNIX clone since it was widespread. They certainly didn't care for the philosophy.
Also, just to muddy the waters of this awful thread even further:
The Linux kernel in its most commonly deployed configuration cannot even bootstrap itself fully without the assistance of user programs (init and udev). On any Unix like kernel, death of the init process will induce a kernel panic because a root process must always exist.
If you define "operating system" to mean only that software which is always running and required for all other programs to function, does this definition not necessarily include "user programs" like init? Do combinations of the same kernel with an alternative init daemon then constitute different operating systems?