They didn't enforce payments. I remember they asked me money but the app still worked when I didn't pay. I was surprised, so when they asked me money again many months later I didn't pay again to see what happened. The app kept working. Months later they asked me money again and this time the app stopped working. But back then whatsapp was not as popular as it is now. I was still using SMSes with most of my contacts (my plan had more free SMSes than I needed) and Viber was at least as popular as Whatsapp and I remember that I was using at least another two or three instant messengers with different people. So I didn't pay and I waited to see what happened. The other messengers where free. After a couple of days whatsapp started to work again. So it was official, they bluffed about payment and I knew that I could ignore their future requests. They cared about growing and not about being profitable. The $1 per year business model wasn't for real.

My guess is that if they enforced payments there wouldn't be any whatsapp now. We would be writing about one of their free competitors from that time now.