When Sprint introduced the HTC EVO 4G smartphone last year many prospective customers were unhappy the network instituted a $10 monthly "Premium Data" charge. Sprint responded with an explanation that the EVO 4G, the carrier's first phone with 4G capability, could use more data bandwidth than other phones and the monthly charge was necessary to continue offering an unlimited 4G data plan. We can expect to see more unhappy customers, as Sprint is extending the Premium Data charge to all smartphones effective January 30.
The explanation given by Sprint remains the same, the $10 charge is necessary to continue to offer unlimited plans at the competitive rates it charges. The new charge does not affect plans on data cards, as they are not truly unlimited; 4G-capable cards with unlimited 4G are subject to a monthly data cap when the device drops down to the 3G network. That happens regularly as the Sprint 4G network is restricted to major metropolitan areas, and once you travel outside those your data card is likely to drop down to the Sprint 3G network, which is capped. Smartphone plans are unlimited on both 4G and 3G. You now pay the $10 Premium Data fee on all smartphones, not data cards, to provide that unlimited service according to Sprint.