Simply put because it uses licensed spectrum (1900Mhz) and can only operate in areas where Sprint actually has a license to use said spectrum. If it didn't restrict it to areas where Sprint owns licenses it could interfere with the actual license holders. T-Mobile HotSpot at home uses WiFi, which uses unlicensed spectrum so it really doesn't matter where you use one of their routers, however the downside being it's only compatible with certain phones.
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Simply put because it uses licensed spectrum (1900Mhz) and can only operate in areas where Sprint actually has a license to use said spectrum. If it didn't restrict it to areas where Sprint owns licenses it could interfere with the actual license holders. T-Mobile HotSpot at home uses WiFi, which uses unlicensed spectrum so it really doesn't matter where you use one of their routers, however the downside being it's only compatible with certain phones.