Read reddit’s linked description of what a quarantined community is - it explains the answer to your question.

It’s not about people who want to go to fan sites (or don’t want to go) - it’s about misinformation / inappropriate content leaking out to folks who might inadvertently view it and not understand the rat hole they’re about to go down.

Putting the subreddit into the quarantine just means that there’s an extra disclosure and opt-in before viewing content from that subreddit (and content won’t be shown in popular, /r/all, etc.)

The subreddit was in violation of Reddit's rules. I can only guess that they finally did something because the mods were too slow to respond this time.

The problem is that they violated Reddit's rules by posting threats of violence against Oregon police & public officials.

That would be nice, but they come into normal subs with detached from reality troll comments and disturbing "opinions".