It really is more cruel to say that because you, in a first world country, can pay 1 dollar much easier than they can pay 1 dollar, you should decide that they're not allowed to pay with their data if they desire.
We're talking mind-boggling levels of poverty. How are they able to even pay for a phone and mobile subscription? Where do they live? How do they eat?
It sounds to me like a made-up sad story used as an excuse for rich corporations. If there are people that poor out there, Facebook could charge the rest of the world while offering the service for free to those that really can't afford it.
Even if you use whatsapp on a $50 phone (which you use for say 5 years) with a $3/month plan (which seems incredibly cheap), you have amortised costs of $46 per year. Increasing it to $47 to get whatsapp does not seem as unreasonable as you suggest.
Malawi is also an _extremely_ poor country, for reference Bangladesh have ≈ 5% smartphone usage with a GDP of ≈ 4000 USD per capita (PPP) while the GDP of Malawi is ≈ 1000 USD (PPP) per capita.
I would argue that not paying (but having the means to actually use whatsapp!) is more of a choice, or lack of payment infrastructure, rather than not having the disposable income to spend 1 USD per year for the service.
If India and Malawi and whatever other country can't pay for WhatsApp, they don't get it or Facebook makes it free just for them. Or they find a way to make the ones that can pay do so.
What's the big deal?