Communicating what happened and how they would do that is an interesting problem. Knowing which machines are infected is simple because they were contacting the control servers regularly. Knowing where the machines are and who they belong to is not. I suspect it would a lot of work and expense to discover the physical addresses of all the machines to communicate officially outside of leaving something on their computer, and writing software to leave some kind of official “calling card” behind that would inform the user what happened is neither trivial and would likely also upsetting to people. Most would assume the message itself is some kind of scam or mal-ware itself. I’d personally still want to know, especially since I might have the actual mal-ware on backups or other infected machines that are offline, but I’m not altogether surprised if they chose not to inform the users at all.