It's true that in theory, we can define any XML message (and attachment + properties) as a JBI FAULT.
But, there is no official binding JBI that we support on the WSDL exposed in the bus, with some rules about how describe a JBI FAULT that can be generically handled.
Thus, we commonly use binding SOAP, for ease SE BPEL and BC SOAP manipulations.
EG, if a SOAP fault is returned by a JBI component when a invocation comes from the BC SOAP, the actor is relayed in the SOAP Fault returned by the SOAP. I think it is interesting to know the component name of the origin of the fault.
To go further, we can define so rules about how handling and define a JBI Fault in Petals ESB. Open discussion, for my point of view, i don't know!
Why faults should be SOAP faults ? IMO, The SOAP faults should be returned by the BC SOAP to its client, and into the NMR we should have only fault not using SOAP message protocol.