Essentially, the person doing the massive downloading is using the line MUCH more than the seldom surfer. Why is it fair they pay the same amount every month?
To put it in terms of water lines, it is because your ISP is selling you the pipe, not the water. If you want quick access, regardless of how often you use it, you have to buy the infrastructure to support it. People who use it more often simply make better use of their investment.
Since your ISP is not providing (the vast majority of) the data you are downloading, their is no reason they should be charging you for how much you download. If everyone stopped using their connection for a week, it would cost the ISP no more or less as the infrastructure installation and maintenance is already a sunk cost.
This total amount of bandwidth is then split between a number of people that must share it. Not everyone uses their internet to its maximum capacity at all times, so they overbook their lines to cut down costs.
This is true of other communications networks as well and I fully support it as a method of reducing cost and making more efficient use of resources when implemented correctly. Take telephone lines as an example. Under normal conditions, there is a rather small probability that this "overbooking" will cause you to drop a call or cause your voice to cut in and out. (We'll ignore mobile phones for the moment as most of their problems stem from their data services.) Conditions where you don't get your paid for service are rather rare and considered the exception.
The problem with many ISPs (including data services on mobile phones) is that they use reduction of service as the norm rather than the exception. If they can't support the rates they advertise, they should change the advertisement. If there are to many people using higher data rates on their network to support, expand the network, lower the data rate, or limit the number of users allowed to connect at that rate. They could even post the number of spots and charge a premium to be one of the "elite" that gets to use the higher rates. Then they could use the premiums to expand their capabilities. Making this progression public would likely be beneficial as customers would feel like they are getting more for their money.
A compromise to the current situation would be it ISPs described the rates your package gets up front as dependent on the number of users vying for bandwidth at a given time. Throttling specific protocols is effectively dictating how you can use your connection. On the other hand, two customers paying the same amount should both get equivalent service (in this case data rate). To address both issues it would make more sense to divide the data rates based on active physical connection (they could use the ISP distributed IP addresses), rather than logical connections. This would allow ISPs to shape traffic to more evenly between customers during high usage periods. Obviously, some extra considerations would go into traffic shaping for customers with multiple IPs, but the basis is the same.