Apple users are finicky and love aesthetics. That's one of Apple's/Ive (before he left) major selling points was simple beauty. A lump on the back is not beautiful, simple, clean or elegant.
To say half of Americans are finicky and love aesthetics because there's no other reason anyone would buy one is a bit of a stretch.
(I'm probably buying an iPhone this year for the first time ever, and I don't really care how it looks, or how good it's photos are, and I'm definitely putting a case on it.)