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.
Extend this logic to actual comments and ask yourself how quickly this would descend into 4chan.
Whether you like it or not, a vote is a much expression as any type of reply. Why is it that a button that says “I dislike this post” should be protected while a comment saying the exact same thing should not?