@jcj - The BEA cannot start an investigation if the Japanese and Taiwanese don't want them to, nor can they involve themselves in an ongoing investigation without being invited to do so.
@TD - The NTSB investigators were certain there was a problem with the rudder somewhere, but were stymied by the fact that they couldn't precisely prove what the problem was. As for the Boeing viewpoint (prior to PCU thermal shock discovery):
The Rudder Story
The Boeing View
Charlie Higgins, a Boeing vice president who heads up airplane safety and performance for Boeing, said the rudder control units from the 737s in the Pittsburgh and Colorado Springs crashes were both thoroughly examined as a part of the NTSB's accident investigations.
...
The company points to pilot error in the Pittsburgh crash, suggesting the pilots may have mishandled the plane in reaction to the turbulence, with the first officer inadvertently holding the left rudder pedal to the cockpit floor as he and the Captain pulled back on the control stick to break their plunge. They blame a rogue wind in the United Airlines crash and a mis-rigged yaw damper for the Eastwind incident.