It's not data at all. You are paying for a share of the larger fixed-size bandwidth pipe to other networks. So what you call data is effectively transit or your share towards the cost of carrying large volumes of data from say a server to your device. Forfeiting "data" is not how the operator makes money per se. They do however make money on oversubscription. So if the overall pipe (a simplification) costs R100million per month and each of 1 million customers pay R200 per month, then the operator effectively brings in R200million - this covers the cost of the pipe and other things (salaries, marketing, promotion, amortisation of network equipment then profit for shareholders etc).
The per-megabyte system of paying for "data" is just a convenient and "fair" way of billing you and limiting your usage to a fixed amount.