> Users that have more than 500 karma in quarantined subreddits outside of The Donald will not be allowed on the list of approved applicants [to be a moderator]
This is only for being a moderator. Simply upvoting posts (that don't break any rules) in quarantined subreddits does not get you punished.
> If the content that people are upvoting is bad enough to get banned over shouldn't it just be removed from the site and not left up to upvote?
What ends up happening in this subreddit is that the bad content doesn't get reported/deleted, which is what leads to the subreddit being quarantined in the first place. The whole point of a quarantine is exactly to push people to report bad content, and acts as a warning. This new change makes sense as far as pushing people to report said content and not keep promoting it.
I also don't think the content is being "left to be upvoted", but rather when found, they will retroactively flag anyone who upvoted it instead of reporting it. But the problem is that the content often doesn't get flagged/deleted until it gets to the top of the subreddit. This is the problem they are trying to fix with this change.