The mod banning these users is the same mod who made the posts they downvoted. This is mod abuse, turning the downvote button into an auto-self-ban button.
The message is “If you disagree with me, you will be banned”
Monitoring and banning users for using lemmy as intended to signal boost your opinion should be grounds to have all mod privileges removed. This behaviour undermines the integrity of the server and the wider fediverse.
Absolutely, it is rare. But people do it. As I’ve said before, I banned 5 people on the original !television@lemm.ee instance for just downvoting posts repeatedly. No pattern. None of the accounts were active on the community in terms of posting. Some of the accounts had never even posted on the fediverse - they were simply downvote accounts that purely existed to vote negatively on content.
Piefed has much more control here. People can easily just block communities though.