I might be wrong, but so far my imperssion is Whatsapp trying to be proprietary and make money from selling the app, unlike other IM, especially those from big guys, are quite open and cooperative.
I kinda feel bad that it got popular but not other more open IM.

All data has to go thru their servers and they are selling the service, I doubt you can make a free app without being blocked by them sooner or later.