No TOS needed, there's already international copyright law that explicitly disallows this. With a system like this, you are _embedding copyrighted content into your website without consent_. If it was a single quote, a comment, or maybe even a screenshot it might be exempted, but you cannot just copy/paste someone's full content into your website without permission.

PS, Hacker News might not even be in the position to officially allow this, similarly to how a generic OSS dev cannot just relicense a project without getting consent from all contributors (or a previous CLA).