I think the thing here is why are they being contacted about a private message

Because WhatsApp scans messages for when "imminent harm exists".

If anything only those party to it should possibly report it

That's not how Snapchat works.

It stands as it does as a cautionary tale about using any service like this

Absolutely.

because private conversations should not be looked at in a public announcement kind of way

It wasn't a "public announcement", it was forwarded to law enforcement.