Alright, hot take: This isn't the case, especially on Lemmy/kbin. Since upvotes/downvotes don't influence sorting, and scores aren't totaled into a single number, there's not really any downside to downvoting someone for just being a shitty opinion or something. If anything, it's actively informative that this-many-people thought this guy was dumb, but _this* many people thought it was good.
Up and down buttons started off as "like" and "dislike" buttons. That is what it's always meant, and is likely to continue being how most people use them
It's for content that you don't want to see. The reason is your business. I think you're confusing the etiquette rule that downvotes are not for opinions you disagree with, because we want people to be able to discuss different viewpoints without the hivemind casting them into the ether.
If there's content you don't like it is perfectly valid to downvote.