The short answer here is: it’s a temporary measure. It’s a legacy from bootstrapping the network. It’s phased out and will not be done that way going forward; all the nodes that are paid are currently in the process of being provisioned and are planned to be added to the network in the course of this year.
So why did we do this at all? The answer is: Bootstrapping a decentralized system is hard. The IC needed nodes to run on from day 1, decentralized to the best-possible extent. Independent NPs accepted the risk of investing into node hardware and data center contracts prior to the network even being operational. It’s those nodes that stem from these pre-genesis times that do receive rewards right now, independently of whether they are actually contributing to any subnet.
Now there are of course questions like: Why are so many of those nodes not up and running after one year? To which IMO the answer a mixture of the IC anyway not running at capacity right now, and consistent improvements of the rollout process that make the process much easier now than it was a year ago.