Whilst this does seem a dubious/late-in-the-day decision, some perspective is also required.
My recollection of entering the site last year was that nearly everyone (90% for sure) entering at the time I did had prepaid.
I clearly remember seeing a small half-size portacabin to the side of the main entrance, which was the place to buy "on the day" tickets, looking very quiet.
My strong suspicion is that accurate records are being kept of advance Saturday sales - and it is getting full (or as much as car parks that aren't under water can take).
I say this because when I bought my tickets in Barclays the other day, the cashier showed me the record she was having to fill out and submit to RIAT of tickets sold for each day.
Despite my observations from last year, I can see there might be some trouble over this
- good job there will no doubt be lots of heavily armed plod around.