The app is indeed free to purchase, but now follows the same pricing model as all the other mobile platforms in place already offering the app.
This means that it is free to purchase, and use for the first year, but will cost an annual renewal of $0.99.
So of course $0.99 is nothing to fret about, but it does mean that you cannot keep using your iTunes account that has no payment method linked to it, unless you utilise a voucher of course.
WhatsApp makes no reference about how they will “recover” this cost if you do not have a payment method linked to your account.
I would have to imagine that they would deactivate your account until you do actually pay for it after the 1 year free.
If you are already using WhatsApp, then nothing changes for you, as we become exempt from the pricing model.
That’s it. Short, sweet, free…..Enjoy!