This plus not to mention Sprint's service is not the greatest in some areas. Their support for Windows phone is abysmal as they only have the Samsung ATIV S Neo and the HTC 8XT which both are available online only. My parents have sprint and my mom is fed up with her LG Optimus Pro killing her battery and doesn't want an iPhone like my dad.
Any AT&T branded phone does not need to be unlocked to work on an AT&T mvno (airtime reseller).
However any mvno branded phone does need to be unlocked to switch between mvnos. Weird. I've heard Cricket (on AT&T towers) branded 1320 does not cross over to Straight Talk on AT&T... Asks for an unlock.