Two points on that. First, it's been Summer here since early May (kids out of school and everything). By August, we're ready for it to be over.
Second, HHN and Howl-O-Scream do exactly what theme park junkies want. Once a year, 8 or so new "E-ticket" attractions. These may not be omnimovers, but in general, the themeing rivals PotC or HM. They disappear in a month and a half, and that exclusivity only ups the coolness factor.
In other words, spending on new attractions drives fans to visit the parks more (and spend on drinks, food, jello shots, T-shirts ...)
Rube price. For the same amount, an AP can get every night but the Saturdays. Even then, most of the normal Universal rides are open as well, so roughly analogous to a 1-day pass.