The design of this feature is insane. “Uncategorized” should mean “no categories set”. This is reflected in the weird UI behavior where “Uncategorized” is treated like a special option that checks itself when you uncheck other categories, and all the other special-case code around the “default category” misfeature.
To put it another way, under the current design, you can have no categories ([]) or you can have some categories (['miscellaneous']) or you can also set null as a category ([null] or ['miscellaneous', null]). Everything gets the null category by default… and null or Uncategorized is… still just another category under the hood. You can even assign Uncategorized along with another category if you really want to.
This is a seriously flawed design. However, I think we are stuck with it: changing this would break a lot of things for relatively little benefit. So I think the right thing to do is to close this petition.