Maybe the driver did see them and felt it was safe to pass, we don't know.  It seems the letter was written with too much emotion and too little factual clarity.  If there wasn't a shoulder beyond the fog line, it seems like a dangerous stretch to ride at 15mph, in a 55. 

Of all the other roads to ride on, why pick such a dangerous section?  (Reminds me of the thoughtless riders drafting wheel-to-wheel on a shoulder-less section of Territorial.)  Don't riders understand how vulneable they make themselves, right or wrong?