You're confusing name space and semantics.  It is indeed in the name space
but the semantics of writing it is to change the state of the writer not
everyone who can see the name space.  Processes owned by different
users can all share objects in the same name space.  However reading or
writing those objects can have different permissions and results for
the different processes.

If I where to unbind /dev, all processes in that name space
would see that object disappear.

The discontinuity/incongruity in Plan 9 is what an object can see
about the process using it.  Kernel objects (things implemented with #x
for any x) can see the process and can adjust their behavior
on its immediate state (this is especially true of #p).  However
anything that goes through 9P only sees the user's state by way of
the messages sent to it.  It can't tell anything about the particular
process performing an action and can thus only adjust its semantics
according to the user who actually bound the object into its
name space (this action turns to an attach).