Eactly. If it's just like water then bill it like water. The water company doesn't charge me a rate for every 100 cubic feet. They charge per cubic foot. ISPs should charge per GB, or even per MB.
Every utility I have once my usage goes over a certain amount the per unit cost decreases. ISPs got this backward.
They also charge you a rate on the size of the pipe that feeds your house/business. I guess you could think of this like speed tiers. Also, I have never heard of a water company cutting off the supply of water to a home or business for using too much.