Nothing sensible about this at all, irrespective of the reasons (whatever they really are). Its too late now to start saying on a website that its now a ticket only event - not every prospective punter is going to be looking on the website this late in the day. Maybe 2 or 3 weeks ago may have allowed them to inform their audience!
So as has been said, those 10000 people who would have bought a ticket on the gate will probably still make an attempt at turning up, not knowing that they won't get in. Its not going to reduce any of the traffic, and will just alienate a whole group of people for any future air shows, who are probably already seriously disgruntled at the price of tickets.
Cock up of "Spirit of Adventure" proportions I'd suggest.