I can understand why sprint is charging for this. They offer unlimited data for your phone, not every other computer and device you might want to connect through it. If a company makes you pay overage then they should not charge extra for this feature, since you are already paying for a specific amount of throughput. Att double charges their customers for access to their 4GB of throughput. 4GB of 1's and 0's is 4GB of 1's and 0's no matter what is using it. Doesn't matter if you max out your bandwidth completely, cuz you are paying for the throughput.
With sprint they give you unlimited 1's and 0's, for one device. That one device is not continuously using the max bandwidth. When somebody else adds a device to that bandwidth you max out the bandwidth more often, that's why they charge you a little more for that feature. For that they don't throttle your bandwith. When they start bandwith throttling then that charge would be unjustified.