Who decided that’s how downvoting should be used? There is no official rulebook (especially on the fediverse), and etiquette is decided as a group, but there isn’t clear consensus on this.
The technical function of the downvote is to push the comment down far enough that people won’t see it. And so people will continue to use it as a way to communicate that they do not approve of the comment. And telling people to stop downvoting comments they don’t like is trying to enforce a rule they never agreed to.
I sync OneDrive to my Synology as well, another reason I’m okay with the “double” traffic is that my and my wife’s photos from our smartphones immediately get backed up to our Synology, even when we’re away from home.