If someone posts a copyright violation on YouTube, YouTube can go free under the safe harbor provisions of the DMCA. (In the US.) YouTube just points a finger at the user and says “it’s their fault”, because the user owns (or claims to own) the content. YouTube is just hosting it.

I don’t know of any reason to think it’s not the same for written works. User posts them, Reddit hosts them, user still owns them. Like YouTube, the user gives the host a lot of license for that content, so that they can technically copy and transmit it. But ultimately the user owns it. I assume by the time Reddit made the AI deal they probably put in wording to include “selling a copy of the data” to active they want in the TOS.

Now, determining if the TOS holds up in court is of course trickier. And did they even make us click our permission away again after they added it, it just change something we already clicked? I don’t recall.