DOESN'T MATTER IF YOU DO OR NOT!
Smart Phones are expensive to make. If you were to buy a new phone without a contract, it would cost you $500+. Some examples:
GSIII and
iPhone 5.
When you sign a new contract with an American Carrier, you pay a subsidized price. Sprint says, "I'll tell you what, you pay $200 for this phone right now, and we'll get the other $300+ back from you over the next two years." That means you are paying more for your service, so that Sprint gets the full price of the phone. That's why it will cost you money to break your contract. You are paying the rest of the subsidy.
It's also why it's cheaper for SundancePoundKid to pay 300 for a phone right now and not sign a new 2 year contract, than break his contract current one ($200ish) and switch carriers, then buy a new GSIII on contract ($200ish again, probably find better deal, though.).
EDIT: That's why the Nexus 4 is a really exciting phone. It is only $250 unlocked and subsidized. What is arguably is best android phone, you can get for 1/2 the price of the GSII, and a 1/3 of the price of an iPhone 5.