The 4chan/4channel moderation is much quicker to respond to reports than they're given credit for. Any post flagged by users as illegal is dealt with (deleted and the user banned/reported to authorities if applicable) usually within minutes. First line of defense are janitors from the community who do the work for free, anonymously and are hated by the very community they help.

well, each thread basically has a time limit before it gets archived and removed, so the users know to keep making threads if they want to keep their topic alive.

So if someone comes and makes a DMCA claim on a thread, the moderators can just ignore it and wait for the thread to time out or they delete it and the users just make another thread.

As long as the moderators wait a couple hours or so to respond to legal threats, and maintain a semblance of "low moderation" they pretty much have plausible deniability to void copyright. It's sort of genius.