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.