Sprint says the "Premium Data" charge will help the company continue to offer their cheap unlimited plans.
Says the carrier (via PR):
Sprint wants its customers to experience the range of entertainment and productivity possibilities available with today’s wireless technology. While some of our competitors impose overage charges and complex plans, Sprint continues to provide a worry-free, unlimited data experience while on the Sprint network. This is responsible, sustainable and reflects our commitment to simplicity and value.
Sprint’s unlimited data plans, with or without the $10 charge, continue to beat the offerings of our top national competitors, who cap users’ data, charge data overages, and do not offer the unique functionality of Any Mobile, Anytime, which gives subscribers on the Sprint network unlimited calling to any other wireless user in America regardless of carrier.
Smartphone users, on average, use around 298 MB per month in data, but that number has been slowly increasing as more and more Android phones are put into the market.