They, like the WinRAR people know what they're doing. They know the moment they try to enforce people to pay up after their times up- people will start using another fully free messenger. And there are tons and tons of free messengers on the market. So, they can't enforce payment and if they remove the payment aspect completely, they risk making no money whatsoever. Thereby keeping it in ambiguous like this means people stay satisfied and happy thinking whatsapp is free cause of their time extensions meanwhile they will still earn something from the few people in the world who actually pay up before their whatsapp expires.