But once they are made aware, the legal system considers them to now bear responsibility if they don’t take action.
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.