Why using res-sellers, like carriers use re-sellers to sell SMS integration? Distribution and fair-use control. First, companies like Twilio are the natural partners to sell one more channel to customers that already integrate to SMS. Second, partners will help enforce and control anti-spam efforts. If opening Whatsapp API turns it into a spam jungle like email or sms, it will certainly lose its appeal and users will start switching to the next new spam-free chat app.