The work a category preference does can be best described as
When a new topic is created in (foo) category, automatically set its notification state to (bar)
Removing a category preference would not reset all existing topics in (foo) category to a different notification state, it just stops future topics from having their notification state set to (bar).
Mute is perhaps different because it is so extreme, mute means “I hate this category with the fire of a thousand suns, every time you show a topic in this category to me I will kill a kitten.” (I may be exaggerating for effect, but mute at the category level basically means make this whole category stop existing, so vast suppression in the UI.)
This is probably another request for bulk setting of topic data (category, notification state, etc) on a lot of topics at once through the UI. Has come up dozens of times since launch, we need to get to it eventually.