My understanding is that there's a large part of Instapaper is server-side. Since there's no recurring subscription, only an initial fee, the maintenance of the

My understanding is that there's a large part of Instapaper is server-side. Since there's no recurring subscription, only an initial fee, the maintenance of the service for existing users is supported by the initial fee paid by new users. This model would not make it possible to allow the developers of clients for other platforms to keep all the revenues for themselves as the costs of maintaining the service infrastructure are non-trivial.