Data caps are not about quality of service. Data caps are about getting people who need more data to pay more money for it, so that the service provider doesn't have to increase the cost of every customer's data plan. Instead, only the customers who need more data will pay more. The increased cost is a cost of doing business, but they have to get the money somehow. They chose to only increase some people's costs versus increasing everyone's costs. This is what they mean by "fair pricing".