The requirement for helmets in pro-cycling is an interesting subset. There is statistical evidence that the number of pro-cyclist deaths has actually risen since they were made mandatory in 2003 (admittedly we are talking numbers of around 0.5-1 death a year). For example this blog review of the stats from Dave Mouton: -
http://davesbikeblog.squarespace.com/blog/2011/6/21/pro-cycling-and-helmets.html
As for the 100 milers all being lidded up, audaxers ride far further than 100 miles (my top so far is 260, but that's not that far in Audax terms) and there are far more non-helmeted wearers than your figure suggests, and this seems to go downwards as you move through the distances. The accident stats do not show lots of head-injury related deaths however. We also had much lower than 99.9% helmet wearing on LonJOG last year which was 80-130 miles each day for a week, yet had no head injuries.
I do wonder whether sportives insist on them through a belief that they are beneficial, or to make the riders feel more like racers*?
*There is an often quoted joke that Sportive riders 'pretend they are racing', whereas audaxers 'pretend they're not racing'