1) This, and the fact that earbuds/mic come standard with all cell phones nowadays for hands free usage.
2) Understandable that headphones come with mp3 players because their main function is to play music. However, the iPad is used in varied ways similar to that of a computer. Not many laptops come with headphones.
3) Keeps the overall product cost down. Apple's earbuds aren't that great anyway (in my opinion), and I'd rather use my own preferred headphones rather than pay a little extra for a pair that'll end up in a drawer somewhere.