The carrier surely wants to make a profit so charging the real cost is just dumb. There will be a certain degree of markup. Typically the markup is rather high and the fairness of it can be debated. But they all do it so if you want it you'll pay for it. If enough people refuse then the prices will adjust accordingly.
I don't understand how usage would not have any impact of costs and why billing should not be based on usage. The costs to the carrier can be broken down into fixed costs and variable costs. The fixed costs will be there regardless of whether the service is used or not. As an example the cell phone tower or the fiber optic cable connecting the tower to the rest of the network. Whether it is used or not there is cost there. But when it is used there is an additional variable cost. Example, the wages of the person who must maintain it. The more it is used the more that cost goes up. Therefore billing based on usage makes perfect sense. There is an aspect of the billing that is a standard fee regardless of whether you use the service or not. This covers the fixed costs. The usage based part covers the variable costs. Usage very much so has an impact on costs.
Does not your water company base your billing off of a standard fixed amount plus a rate per unit of water consumed? Surely the electric company charges you a customer fee plus a rate per KwH. Mobile and fixed internet providers are becoming more and more just like any other utility therefore it makes sense to bill as such.