I would not call it or consider it a "disagree" button. A disagree button would be like a "like" button. Members names would be seen along with the post. In this case it is the opportunity for some members to anonymously and without justification influence the disappearance of other members posts. Since I have yet to see any firm guidelines on using the button, it can be used on spam, trolls, suspected trolls, members you have an irrational grudge against, posts that have gone off topic, humorous posts you don't find particularly funny, for revenge, and apparently to turn orange kittens to gray.
Well said. It means censorship with no accountability.