No, it's in no way a security bug. Why would it not be entirely feasible for an admin to ban a user at one site but not all sites?

I can see many times where this might be warranted. Say if you have a community site and a business site. You might need to ban them for acting out on the community site, but not want them banned from the business site (since they did pay for your software). Or you might ban someone from a forum intended for children while letting them have access to a forum intended for teenagers.
Converge does not carry across permissions or groups.Has been stated numerous times, but I feel the need to restate it here. In the future, at some point, we may (as in possibly) add some sort of permissions/group support, but you have no idea how difficult this is. Forums do not match up from one site to the next. One site might be Nexus while another is Wordpress and another is IPB. How do you map groups between the three? Does Wordpress even have groups? What about permission masks? Which applications support multiple groups, and how would you replicate that in applications that DON'T support multiple groups? What about the setting to override permission masks for a member? What about if you remove a user's posting rights, should that remove them in other apps? How would you correlate that to a blog, for example?
There's a million and one issues with trying to share permissions. While it sounds nice in theory in a basic environment, once you start taking into consideration all these other factors, and the fact that IPB isn't the only application that can be or will be converged, you can see it starts getting much more complicated.