You are mistaken about the cost structure of the internet.
For all practical purposes, all of the cost for providing internet services at scale is in the last mile. In other words, it would not cost internet.org anything significant to extend the service to cover the whole internet. It makes very little difference in cost whether they backhaul the traffic to their datacenter or their closest peering point.
If the requirement is to only provide a low bandidth service then it is trivial to restrict the bandwidth usage per end-user, even if it provides access to the whole internet.
For all practical purposes, all of the cost for providing internet services at scale is in the last mile. In other words, it would not cost internet.org anything significant to extend the service to cover the whole internet. It makes very little difference in cost whether they backhaul the traffic to their datacenter or their closest peering point.
If the requirement is to only provide a low bandidth service then it is trivial to restrict the bandwidth usage per end-user, even if it provides access to the whole internet.