Why can I not set the YafaRAy material settings in the Blender material interface (F5)?

Because YafaRay materials are different from Blender materials. Many Blender material options have no corresponding option in YafaRay materials and the other way round. Therefore, to avoid the confusion which option was to set where, it was decided it would be easier to have the YafaRay material settings completely seperated from the Blender settings. But: Materials are still created and assigned to objects using the Blender interface. Once a material is created, it can be changed in the YafaRay interface. The textures of a material are still set in the Blender interface, most mappings are supported.