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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 Lemmy are public. lemvotes.org exists, and Friendica and mbin both expose votes, and then obviously it's decently simple (though not super-trivial like those three methods) to set up your own instance and look over all the votes.
You might feel that there should be a special category of "lesser" (you say normal) user that is unable to see votes, even though another category of user is able to. We could talk about that philosophically, but regardless, normal users can see votes. Vote accordingly. The error lies with the Lemmy UI being designed in a way that doesn't make it clear to people that their votes are not fully private.