It's a premium data fee, i.e. 4G LTE even if you aren't in a 4G LTE area. Sprint started charging the fee shortly after they launched the HTC EVO. Like AT&T with the original iPhone they weren't prepared for the amount of data that would be consumed by EVO users.
That ticked off the EVO users because they were the only one's paying the fee. So…shortly after they started charging the fee, Sprint renamed it to a premium data fee. That's where the confusion began. Because Sprint renamed it they decided to charge it to anyone using a smartphone that started a new contract. Unfortunately, a lot of people who had smartphones but not 4G LTE capable phones got caught with the charge. There was a whole breach of contract dustup and Sprint eventually ended up stopping the charge for about a year or so for people with non capable 4G LTE phones.
Earlier this year, Sprint tried again. Only this time they did it on the sly. At least with family plans. They reduced the family plans by $19, but started charging $19 for the second line and above (previously it was free for the second line and $19 additional after that).
This actually hit those with heavily discounted plans (SERO, employee discount, credit union discounts, etc) the hardest because their discounts were based on the cost of the primary line. Effectively, this netted Sprint the $10 data fee because of the reduction in discounts. It also hit those customers that had previously avoided having to pay the $10 data charge.
Additionally, Sprint started charging another $10 fee openly which they actually called the premium data fee. This was on top of the other rejiggering of the plans. It now applies to any smart phone period.
That's how Sprint eliminated discounted plans and long-term customer plans (like ours) that weren't paying any premium data fees at all.
So technically, you are being charged $20 for premium data (at least with family plans), but you wouldn't know that if you weren't a long-term customer because Sprint does not break that down in the billing.
If you aren't already aware of it, you might want to look into the Sprint Users forum. Lots of Sprint employees and ex-employees frequent the place (including techs) that are very helpful.