Looks like they didn't want to go with the "charge $1 per user a year" route.
WhatsApp users to receive adverts - BBC News
Facebook Takes First Step Toward Making Money From WhatsApp Deal - BloombergThe updated privacy policy also paves the way for businesses to send messages to WhatsApp users.
"It will help them generate revenue if they charge businesses a fee to send messages"
How they have changed from:
...you can still count on absolutely no ads interrupting your communication. There would have been no partnership between our two companies if we had to compromise on the core principles that will always define our company, our vision and our product.
https://blog.whatsapp.com/499/Facebook