Honestly the biggest influence, rules-wise is the Great Divide Race. Much smaller in scope of course, but GDR's concepts of no prizes and racer responsibility for route finding are key to making this work.
Also PennDot really is to thank here too. Without the PA bike routes this wouldn't happen. I'm not down with marking a course on public roadways, or making my own map. I have neither the time, compulsion or willingness to open myself up to the legal liability of doing an event like this. Next thing you know I need insurance, so I have to charge people, so I have to register them, then prizes will be next and the whiners will show up.
Which leads me to thinking about the absolute lack of whining from participants (save for the first year). It gives me a warm fuzzy feeling.