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Even if it's technically not infringing, no way Disney would let this slide considering how annoying they are about protecting their IP. Even when they're not legally in the right, they have the money to shut this kind of stuff down (which is why I've never tried to design a Haunted Mansion set lol....would be great for a WASD one-off though!!!)
I designed this keycap set a long time ago, but finally decided to order it from WASD Keyboards. The responses during the design/proof process were super quick, and the total turnaround time for this whole thing was insanely fast (I think about a week between sending in the first design files and having the keys in my hands).
The printing is really good. You can see in the photos that the top of the printed keys have a shiny textured finish, which I believe is a UV protection coat that protects the prints. It's not present on the blanks I ordered. The keycaps themselves are nothing special; fairly thin ABS, but no QC problems that I could see. 
For my design I stuck with EPCOT's 80s color stripes theme for the alphas and mods in the main 60% area. The font is "prototype", a fan made recreation of the one used in Epcot's original signage. The F-row is the icons for the original attractions,  plus Epcot and Disneyworld logos.
I bought the arrow keys separately (in two colors) and didn't need a numpad, so I could use the those keys in the 104 key template to create alternate keys to cover nearly all TKL or smaller layouts. The two problems with that plan are that one numpad key will have a bump, and that there's no 1.75u R1 key for shift, so I ordered that as a separate printed keycap.
The lack of 7u spacebar options, or 1.5u r1 keys means I can't use it those boards, so hopefully they make those blanks available sometime, but overall I'm super pleased, and am already thinking about new designs that might take advantage of the full color printing capability (I designed this one to look doubleshot-able). As the title says: Given the how central customization is to this hobby, why don't more people take advantage of this unique service? >

As a Disney World/EPCOT fan, this design is great. Honestly, I think most people would not be able to design their own sets that look this good and unique, hence why they wouldn't use this service.
Also the quality isn't comparable to doubleshot/dyesub sets. (Not a knock against your set or what WASD offers, just trying to answer your question about why more people don't use this service.)
A shame Disney would put a stop to anyone trying to mass-produce and sell this. Super well done!