I would like to know the reason for the requirement that every resource object must be reachable from an object in the primary data.
I cannot see how a client could take advantage of this additional information.
On top of my head, it helps preventing people to design messy endpoints which unrelated resources. For example, gather all kind of data for the homepage that includes the ten latest blog posts, the local weather, recent comments, etc in a single request.
You have your main data. The included data must be related to it, otherwise why is it there? It seems to just make sense.
Just for fun, do you have a use case for a request that makes sense where the included resources are not related to the main data?