Apple is sponsoring the remodeling and redecorating of subway stations in New York and other big cities. By your logic, Apple gains nothing from that either.
It's called advertising...and brand awareness. Sponsoring a big high tech pavilion at WDW would solidify Apple as the computer and technology leader to people who go to the parks; when they'd return home, they'd be more inclined to pay more to buy an Apple product because they'd have that good association with their Disney vacation.
Apple products are always more expensive so giving people extra incentive and warm-feelings to buy one is a smart move on Apple's part.
My son bought me my first Mac for my birthday last year. I never bought a Mac before because PCs were always half the price, but my son got a great deal and wanted me to have one so we could do FaceTime. If Apple had a presence in Tomorrowland or at EPCOT I bet I would have bought one on my own years ago because I'd have come back from my WDW with Apple on my mind...and if I had the chance to really experience Apple products at Disney and be blown away by them I bet I would have RUSHED home to buy one when I got back from our trip.
I always find myself taking another look at Disney's sponsors when we get back. As I am sorting through my vacation memories, the products I saw advertised in the parks just leap into my head and I check them out on Google and on Amazon.