Yes,it is weird. When I went to Bracknell/Sandhurst earlier this season, they did sell adult tickets on the day, although they also offered a discount for advance e-tickets, so I duly got one. A lot of people were still paying on the day, though.
I can't see it selling it out, with an attendance of something like 900 being my guess. I am sure pushing the tickets like this will help them to estimate and plan for the total crowd, and imagine a cash turnstile for adults will suddenly become available, unless there is an unexpected surge for tickets (whatever that would mean for concessions who were still waiting to buy tickets), but I have no special knowledge, and don't hold back on buying a ticket based on what I say.