I’m fairly sure the obligation to report goes back as far as the DMCA at least. That law gave carriers and hosting providers certain limited liability against user generated content provided they had reasonable moderation policies and acted promptly to take down content either subject to copyright complaints or other legal demands. Basically a you’re ok as a host if somebody does something bad, just so long as you clean up the mess as soon as you become aware.
Hoho, buy how vuld we maintain ze image of ze snooty fenchman with ah rideeculously long cigarette holder zhen?