It may actually be what is intended (I don't know), what I would rather expect, personnally, would be for example that if I write "/kill @e[type=!c", then it would suggest "!creeper", "!cow", and other entity types starting with "c", with the "!" before that.

I would find that much more logical, but I can't tell if that's a bug or not, so I decided to consider that as a bug since I still would find that decision weird.