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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.
Votes on sites like this are an algorithm by way of the masses, rather than what you'd find on centralized sites like yt or the like. It's how the front page gets curated to presumably interesting posts instead of being a random spew of every post made.