Would you say the same for people who give a heart or thumbs up but don't use words to express why they agree?
Personally, I don't always have time to write out an explanation on why I agree or not. There are times it's easier to give a heart (or if I disagreed, a red x, when it existed) and come back later, if I was able, to discuss.
Oh. And is the angry face not "negative"?
Negative emojis are fine and I normally have no issue with them, but some users were literally stalking others [myself included] and red x-ing everything we posted for no reason. Others here know who/what I'm talking about. When asked time and time again for an explanation, they would red x that as well. That behavior is childish and spiteful, and it did lead to some frustration.
The angry emoji isn't abused in the same way because it can also convey agreement - such as on an abortion issue where the user agrees and is showing anger to support the post.