You're paying for a rate, not an absolute amount. The internet infrastructure works on that rate (bandwidth). You don't buy a 10GB switch or router, you buy a 1gbps switch or router. ISPs peer based on bandwidth.

I do understand, though, that by having a data cap they're encouraging users to use less of that bandwidth. However, again, they're just passing on costs to the consumer instead of paying a fixed cost to upgrade their peering arrangements.