User blog comment:Sarah Manley/New Editor Scheduled for Sitewide Release/@comment-26850787-20110904093600/@comment-126761-20110905113442

Ah, [visual/source] categories are now attached to [visual/source] mode - i.e. they switch together. This is to ensure that you can edit category source code, and the experience still makes sense.

So, as you're editing in source mode, so is the category module. (We're not dead set that this is the way it must always be, but it makes sense from several perspectives)