I mean, of course you are correct, but ... for instance I'm kind of put off booking an Aug trip this year because of last year (the decorations, and the several nights MK was closed early). So, I might not be going. Mind you, if instead of a FALL themed party in Aug, it was summer themed, I'm sure I'd be a lot more thirsty to book again. Of course, maybe they have done the math and I am in the minority here, but I've seen the same thing repeated by countless people on various forums, blogs, etc.
So, there might be some initial start up cost, but if they were selling this party from June through Aug ... I'm sure they'd more then recoup.