This was bugging me, but I've got the answer; I was mulling over this during yesterday's ride. It seems clear they are only interested in events, not distance. Surely if they wanted to know your likelihood of finishing, someone who can set of from their home and make it to Paris with the aid of nothing more than a ferry ticket and a pocket guide to Paris is more likely to finish than someone who needs a whole infrastructure to get there.
But it's nothing about the RideLondon 100, it's about marketing. They are going to send a magazine to every person who fills in the online form; they can sell advertising for more money the more they know about us. Already they can tell advertisers that their ads will reach 80,000 100,0000 readers who are all prepared to spend ~£60 on a cycling event; being able to say 40% (or whatever) regularly pay for sportives, and 20% have done an organised ride to Paris will increase the amount they charge for ads.
And of course, that is why they keep the ballot open long after they have more than enough entries and then send the magazine to 75,000+ disappointed entrants who don't want it.