RESTful realm creation in OpenAM is currently quite limited. From the FAQ:
Q. Can I create a realm using the REST API?
A. Yes, you can create a realm using the REST API, but it will not work until you also create the associated services because these are not created when a realm is created via REST. It is recommended that you create a realm using either the OpenAM console or ssoadm.
This is a request to improve realm creation via REST - ideally a new functional realm could be created and configured through the REST API rather than some combination of REST, ssoadm, or the admin GUI.