The original feature was meant to be used to flag plagiarism and explicit content, and it was a feature that you needed to go to the settings of a particular post/comment to trigger. It worked slightly differently than how it works today. You didn't have 100% down vote mana. Your voting mana, as a whole, was directly affected by down votes you made.

Today people use it to enforce some arbitrary rules about tribes and Hive standards that are determined by the whims of individual groups or people with lots of Hive Power. There is no central order, and without that central order, these organizations and individuals can manipulate rewards at the whim of their own desires while coming out publicly with a façade that they are doing this for the good of the community.