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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.
And it's a lot easier to notice and act on bad behavior when activity is public. Maybe on a centralized service that can afford full time moderation staff, you could restrict that information more effectively, but considering the fediverse is community driven, I think this is an effective choice