I don't really see why WhatsApp would care, because once you have developed the interoperability, audited the apps, and done all that, it doesn't really cost WhatsApp anything if a user is using that app. They lose no profit, doesn't cost their servers any more than their own client would, etc.
WhatsApp makes their money from the business clients/apps (which aren't covered under this, which I think is fine by the way).
So why care where a user is on the planet? I just don't see the business reason for this. Maybe I'm missing something?