What makes you think these "mobs" used WhatsApp for its security reasons? I'd expect it's far more likely they use WhatsApp because it's the most popular messenger in that part of the world.
Remember Nextel direct connect? It was known that those communications weren’t tappable initially, and for a time every street level drug salesman had them.
The way WhatsApp was used in e.g. Myanmar included group chats involving dozens of random acquaintances - basically the equivalent of gathering in a town square to gossip. Needless to say, there isn't any security to speak of in such an arrangement. Nor did anyone ever complain that WhatsApp encryption is the obstacle. It's the part where everybody can gossip to everybody, with transparent scaling, that enables flash mobs - and it could just as well be a plain text SMS otherwise.
Remember Nextel direct connect? It was known that those communications weren’t tappable initially, and for a time every street level drug salesman had them.