This is an interesting issue to me, I can see both side’s logic.
First thing that comes to mind is some kind of pop up or warning that the topic you are attempting to create is going to end up somewhere else other then the category you are currently in and the reason for this is you are not allowed to create a topic in this category…
It occurs to me that how it currently works might be a good way of having topics started in not the best or most appropriate category.
And of course the op could use css to hide the button or change the text, kind of hacky but
every community is different and I can see this as confusing some less savvy users
I can see this request being important under these circumstances