Sprint also uses the restricted sale of handsets to lock people in to their service.
If they didn't have the ability to keep people on two year contracts by selling them devices that only they can sell, they would be forced to compete in the service-only market. Then, only the best provider would survive and thrive! You would see them offering better service at competetive rates (I think Sprint's rates are already pretty competetive) to keep customers instead of making them purchase discounted (often obsolete) hardware and forcing long term commitments.

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