Every non-trivial API has a data model, and API stability requires that it be as carefully designed as any database, unless you are going to start rolling incompatible API versions off the photocopier, which is no small task.

The entire idea of an unanticipated JOIN is beyond the ken of most APIs as well, unfortunately. For an external API that may not be much of a problem, but for an internal one you might end up creating a new de facto schema with a new query language.