I wonder if a faded out version of the button would be a good option. If they have permission to post, it shows the regular Post Topic button. If they don't, it shows a faded out version indicating that it is disabled. If they click on the disabled one, it would still go to the permissions page saying they don't have rights to post.
This is similar to how Windows has their menus configured. If the menu item is enabled, it looks normal, if it is disabled, it still appears on the menu but is greyed out.
I think this method would satisfy both camps and stop the questions about why the Post Topic button appears even when they can't post.
