So there is upstream spam filtering. Thiss seems to contradict what you said: "There is no administrative ostracism. Nobody gets kicked out by brownshirts for expressing their view."
Even if this stupid idea were true, in fact nobody prevents Usenet spammers from reaching those who want to be reached.
If you think that spam is a form of expression, and you're interested in it, all you have to do is post a few articles and use your real e-mail address in your From: header.
Then no overbearing NNTP server admin will stand between you and your spammer friends.
Spam is an expression of views.