Can it be a money issue? Remembering that WhatsApp is not free, the 1$ for a year of service should be how they make money (I think they did a very good job there because no one that I know remembers that). Maybe the on-line phone solution suits them technically and economically, because they don't want to provide you with an alternative client that you might use for free. As they say this is simply and extension of the phone client.