If the above definition of Usenet sounds vague, that's because it is.
It is almost impossible to generalize over all Usenet sites in any
non-trivial way. Usenet encompasses government agencies, large
universities, high schools, businesses of all sizes, home computers of
all descriptions, etc.
Every administrator controls his own site. No one has any real
control over any site but his own. The administrator gets his power
from the owner of the system he administers. As long as the owner is
happy with the job the administrator is doing, he can do whatever he
pleases, up to and including cutting off Usenet entirely. C'est
la vie.