Bug tracker
Subscribe to "category" for email notifications (fix completed in vcs)
I suspect either (1) the system shouldn't give the user the option to subscribe to a category, or (2) it should give the option, but it ought to subscribe the user to all the forums within the category so that the user actually receives notifications of new messages within the category.
Since it goes "Category --> Forum --> Topics --> Post" it is useful to be able to subscribe to a category in order to be notified of all the posts within all the topics of all the forums within that category. So when the user subscribes to a category, the system should say something like "Are you sure you would like to be notified of all the posts within all the forums within this category" and the system should warn the user that it would override any individual subscription settings they had previously for the forums/topics within that category.
Comments / History
The subscribe to forum feature is, AFAIK, only for the current forum, i.e. no subforums. So, I'm sure that if the category was a regular forum, the subscribe would work fine for posts that were made directly in the particular forum (category). However, since it is a category, there will be no posts made, so you won't receive any emails and subscribing to it is pointless.
I hear your argument for the usefulness of subscribing to an entire category (or, in general, a forum together with its subforums), but that aspect of your "bug report" firmly lands in the feature request department
For now, I would say, probably the display of the link to subscribe is (incorrectly) tied to the fact whether a topic list is being shown, which could actually be "just" a list of active topics within a category, in which case the subscribe link should not be shown.