It would be convenient for a non-subscribers-can-post group to be able to prevent known spammers from posting repeatedly. My guess is that banning an address in such a group does not accomplish that(?).
Actually, it does.
I set my test group to allow non-subscriber posting, and banned one of my test addresses. A subsequent email from that address was rejected ("bounced"):
Remote-MTA: dns; lb02.groups.io Diagnostic-Code: smtp; 500 You are not allowed to post to that group.
