There is no prize money, no medals. Let riders choose whatever bike they want for the task.
Agreed. There are precious few events for non-traditional HPV riders (some of whom may not have the option of using DF bicycles) to compete in already. You can always nerd about who was the fastest velomobile, recumbent, DF, fixed, etc. when analysing the results.
I like the AUK model of anything human-powered and road-legal goes, but with special awards for those using some specific technologies. I suppose the geographical nature of long-distance riding in the UK makes for a more level playing field
[1], which is why we don't have an award for unfaired cycles or DF bikes.
A categorisation system makes more sense to lower the barriers to entry when it's primarily an engineering competition, as in HPV racing.
[1] Or less level, in the geographical sense. A typical UK audax has too many of the wrong kind of hills to gain much of a speed advantage with recumbents.