I am not so sure that is the reason for WhatsApp crazy validation. I feel that the reason for that was that it represented a danger for Facebook and Mark wisely acquired at any cost.

WhatsApp is big outside of the US because (at least in Panama, were I was born) text messages are not included in the mobile phone plan; however, you can get a data plan for a monthly fee and keep in contact without additional charges. In the US most people use text messages to communicate.