In the situation where users only have acces to one topic, but there are several topics they can acces: The 'post in' menu is still there, when creating a new post. The user have only one choise, så the menu makes no sense.
Do you talk about topic types?
If user has access to one topic type only, he/she could not see the "Post in" selector.
Please provide us more info where in your project (forum/screen) we can reproduce this situation.
One more time. Now as images instead of files. users point of view:
We've found some situations when this field doesn't hide even if there was only one value. Now it is fixed.