Bangs head to wall
I remember thinking this is unnecessary. I am not sure which version of Moodle I was looking at, since now there is the "include subcategories" tick, which may be required by some users.
Tim: how serious would you evaluate it to be to be incapable of changing the category of a random question?
This does not really fit the model of the new UI anyway, so I will think about this.
I will solve this some way or another.