Apple loves ownership. Devices are always 'owned' never 'activated,' people are 'owners' not 'users.' The most basic form of customization on any iPhone is the wallpaper - background and lock screen. I'd wager that over 90% of iPhones have a custom lock screen of some sort, whether it be a personal photo or just a different image from the default. In iOS 6, your wallpaper was trapped - hemmed into a square by the semi-transparent time, and opaque unlock slider. The small square in the middle may have been yours, but the screen was still definitely Apple's - it belonged to the major UI elements taking over the screen.
In iOS 7, this restriction on the wallpaper is gone. The background spills out, spreading across the entire screen. Any change fundamentally alters the character of the phone. Every iPhone with a custom wallpaper is instantly unique - identifiable by anyone at a quick glance. The first thing anyone sees when they wake up is their phone, and it is now uniquely their own.