Tom. I concur with the facts you present. The user of Creo can not as far as I know define the internal reference frame of the geometry kernel.
All 3D modelers I have seen have an internal reference frame that may or may not be accessible through the UI. It is all relative to the frame of reference in this context of geometry creation and import/export.
Change the reference frame to one of the designers choice for a given task by creating your own (features). I use different csys on native and import data quite frequently and it is part of using CAD. Creo has the functions and flexibility to establish a reference frame of your choice and use it appropriately as design intent would dictate.
One way to think of it is a real world analog of spatial orientation. If you are looking at an object in space and you need to see it from a different perspective you either have to change your location wrt to the object (line of sight) or reorient the object relative to your current position. The view orientation tool in Creo and saved views allow you to view the object from different perspectives but do nothing to alter the internal reference frame of the software geometry engine or any other csys for that matter.
This is done in the code with linear algebra and application of linear transforms based on the designated reference frame (i.e. import/export). You can investigate this using the transform option in the measure tool to obtain the transform matrix between two cartesian frames in the model. I use this often to create a new csys needed to position/orient models that do not have a common ref frame that is usable for design/analysis within Creo.