That's a brilliant business model. They get all the money up front, get to invest how they want, and if there is a price increase, it probably will be less than inflation, and almost certainly less than their investment returns on your upfront payment.
It occurs to me that another possibility is that the 10 year limit does not apply to Network Solutions. Network Solutions was a registrar for something close to a decade before ICANN was created. It's possible that there is some sort of grandfathering that lets them operate under pre-ICANN rules.
