I can understand that.. It's a Trademarked and Copyrighted image.. If they allow someone to use it as an avatar... They open themselves up to a possable Lawsuit ...
As a former moderator for several sites, I cannot understand banning as a first response to a minor problem.
There are other steps available with escalating consequences for dealing with forum users that need addressing.
1: Ask the user to change their behavior. (Please change the logo.)
2: Demand the user change their behavior. (Change the logo now, or it will be removed.)
3: Resolve the problem using administrative tools. (Your logo has been removed. Choose a suitable logo at your convenience.)
4: Suspend the users if the problem behavior continues. (Your account has been suspended from posting for one week for willful violation of the site rules.)
5: Ban the user accounts for repeated violations of site rules. (The site has rules covering the disciplinary process, right?)
6: Ban the problem user by email address/IP range/client ID/and other technological means. (For when the user just makes more accounts to evade a ban.)
As you can see, there are multiple options for dealing with problems. If the other site's moderators aren't using all their available tools, well, it's their loss. It will cost them more than just the occasional user and the site will be poorer because of it. That being said, no mention was made of whether the site tried any other option other than banning. Perhaps they did give the users a chance, perhaps not.
While banning is sometimes necessary immediately, it is a rare situation from my own experience.