Are you meaning banned or deleted(permbanned) friends? Cause I have noticed that when someone gets deleted there friends still have them on there friends list.
You can remove your perma-banned friends manually if you so desire, but some people may wish to keep it there.
If you are temporarily banned, there's no need for this to occur. The way that you can tell if someone is perma-banned or not is if you go to their page. If you can still see it, it's temporary. If it comes up with an error, they're perma-banned.