I would shift the blame back to Reddit here, because they exposed data to mods that are completely unrelated to the subreddit. Another problem is the non-existing appeal process.
GP mentioned that he "commented a couple of times", which means that his participation in the other subreddit is public knowledge.
If that other sub is so bad, then Reddit should ban it outright; why recommend it to users only to get them banned in other subs?
Because reddit is generally hands off when it comes to moderation. It generally doesn't replace moderators nor punishes moderators for their actions.
>why recommend it to users only to get them banned in other subs?
probably because the recommendation algorithm isn't smart enough to figure that out.
Most larger subreddits are managed pretty poorly.
Most of the big subreddits are co-opted by toxic, power-mad moderators, or completely devoid of moderation entirely. A complete purge & reset wouldn't be a bad idea.