Single-day passes are mostly sold to "fill in the gaps," which is why they are sold only at the door.
If someone comes who hasn't planned far enough in advance to preregister, then we have no way of knowing how many days they will be there. Single-day passes help us to make sure our attendance number roughly matches the number of passes sold. (You wouldn't want to get turned away before we hit our attendance cap, right?)
They're also a good last resort. Sure, buying three one-day passes is horribly spendy... but once the three-day passes run out, it's better than having no options at all.
Avoid all of this mess, kids. Preregister. ^_^