Apparently, iPhone developers like it when you use headphones. It’s probably because they have taken the time to mix the sound and music to perfection, and they want your entire focus when you are tapping away at the screen on your phone. They want you, the player, to devote as many of your senses as possible to the tiny four and a half inch screen resting in your palm. They can't do anything about smell and taste (yet), but vision, hearing, and to a lesser extent, touching, should belong exclusively to them while you are investing your time and emotion into their main source of income.
The other possibility, is that Apple is encouraging these developers to promote the use of headphones, so more people ignoring the world in favor of their iPhones, pretending not to care about anything (i.e. cool people) will be associated with thin white wires hanging from their ears.