At various times, most social media platforms have received criticism for alleged failure to prevent distribution of copyright-infringing content. Few, however, have been threatened with widespread blocking more often than Telegram. In a row that seemed ready to boil over last year, Telegram was given an ultimatum by the Malaysian government; come to the negotiating table or face the consequences. A Malaysian minister now says that Telegram is ready to fight piracy.
And the action Signal can take is pretty clear: “Okay thanks for reporting, feel free to file a lawsuit against Alice and or Bob instead, have a nice day.” Remember: even if Signal had Charlie’s credentials to view the chat, unless Charlie is an admin of the chat Signal can’t do anything other than log Charlie off the group. Plus each participant still has their own message store. So by this point Signal has complied with the law. It’s literally Section 230.
Not sure why you’re citing US law when we’re discussing foreign govts. Also the obvious thing signal can do, that most complainants would probably expect as a minimum, is banning their accounts and closing the group.