I know , they cant force you to buy another one of their phones that can only be used on their network.sprint is evil!
This is not accurate at all. Boost is wholly owned by Sprint and is carried on the Sprint CDMA or iDEN. There are many many instances of people activating a Sprint CDMA ESN on Boost.
the reason why is because Sprint likely subsidized the purchase of your phone to be used on the sprint network. That's why your phone was much cheaper when you signed a contract than if you had not. That contract is to ensure they will see money back on that investment (i.e. the subsidy). They're not evil, they simply expect you to uphold your end of the bargain. This is no different than any of the other carriers.
If you're no longer on contract and want to switch your phone to boost, what's the point? You're off contract both ways, correct?