https://www.mbm-law.net/insights/premises-liability-for-thir...
Here's a more nuanced example discussing liability to foreseeable harm by a third party on a business's premises:
https://www.supremecourt.ohio.gov/rod/docs/pdf/0/1995/1995-O...
It is of course differently illegal to use the mail (in the US at least) for criminal activity.
Another way to think about this is, if this liability didn't generally exist, why do platform providers require a special protection?