I think an important distinction here is that in this case, Bell would charge the small ISP or alternate ISP, such as Primus, per user. Rather than using the analogy of pipes, you could say, “All the users of Primus use this many million gallons of water. Primus, you buy all that water and sell it to all those people. No individual person can take more than one gallon per day.” That's where the cap comes in. The right thing would be to say, “Primus, you have to pay for all the gallons of water your customers draw”, as opposed to saying, “We'll sell it to you, but none of your customers can use more than one gallon of water per day.”
The last and most important distinction is that there is a finite amount of water on earth, but there is not a finite amount of Internet. The Internet is truly limitless. It's about the thickness of the pipe. That's why, to my earlier point, it's very difficult to price a gallon of something that is infinite.