Any park or attraction can be bought out for a day at any point - if they are willing to pay the price

It's a business at the end of the day and that's how business works.
Whilst it is unfortunate for guests who may have wished to visit on that day, the park have at least put information out to advise it is closed.
These kinds of buy out events happen at all the parks at any time - though most of the time are unnoticed due to being after the main season
(Alton Towers had one last Sunday and Thorpe has one coming up too )