Why does posting to gmane's usenet mirror want login?

["Joe(theWordy)Philbrook" <jtwdyp ttlc net>]

That is when I just did a reply (as a followup) to a message retrieved from
gmane.comp.gnome.apps.mc.general a minute ago it wanted me to login to
something...

Normally when I post a followup to a gmane mirror of any other mailing list,
it doesn't ask for a password, though the first time I do so for each
"group" gmane wants me to confirm I'm human by replying to a challenge
e-mail. Then if I can send e-mail to the appropriate list, the post gets
there.

But this was different. 

HOST: sn-radius.us.supernews.com (INSECURE)  ENTER LOGIN NAME [jtwdyp] :

And when I didn't login I got:

[Error posting message: 480 authentication required]

To send the message to the list I had to use my isp's smtp to send it 
to Mc gnome org  instead of posting it to the usenet group
gmane.comp.gnome.apps.mc.general via news.gmane.org/nntp

Any idea why that is???