Specifically, because if they outline it as a violation then they can deny responsibility for any harm that does happen. If you tried to act like a smart-aleck and "jailbreak" it with a prompt from Reddit, they need an additional layer to cover their ass legally. Hence, the TOS.

This is pretty common on most social media, too. Horrifying and delirious posts can go undeleted for years, but then the companies can deny liability by saying that it was never allowed on their platform in the first place.