DISQUS

I agree with your premise, but I don't believe this this the reason the ISPs are doing this.
Fundamentally, the ISP operational cost model is driven by the cost of backhaul bandwidth from the exchanges. ISPs who buy from BT Wholesale pay a per bit charge for traffic volume - those who've unbundled an exchange pay for the links form the exchange to their internet access points. Once the customer is acquired, this is by far the largest part of their variable costs for customer service.
The flat rate plans they sell give the ISPs little chance to realise more revenue from user who use large amounts of bandwidth. This action gives them a legal reason to threaten, and ultimately remove from their network, the users who drive the majority of their costs.
A licensed download service or a charge which gave the users the right to download is their worst nightmare - even more users would drive cost into the business which they could not recoup. The ISPs incentives are universally aligned against your proposal.

How could you miss the opening shot of 'The Player'? Some 8 minutes in a single shot.

The minority of users are not abusing the network. They've bought an unlimited service and are using it as such. The ISPs made an assumption about how much they could oversell their network and got it wrong. At the same time there was a rush for market share and prices fell. Now that the situation they created isn't working for them the ISPs are complaining and maligning their customers.
I get confused by the nonclamenture used in these discussions. These are not community networks run by friends for the benefit of all. They are commercial networks for which we pay. Saying "some users use 90% of the bandwidth" is really the ISP saying "We've underprovisioned network bandwidth". If prices need to rise to cover the services we buy then so be it and that probably means some ISPs need to go out of business.
This same argument played out during the days of dial up. Phone companies companies complained that everyones phone calls were too long for the network to support - they'd oversold it and now it was a commercial problem for them.

With all the talk of the cost of bandwidth, we've missed the point - bandwidth is incredibly cheap. All the UK Telco's appear to have come to about the same conclusion - they would be paid around �£0.25 per movie download (or about 50 cents US) and that would have to include a profit margin. This is the only cost saving this service can realise. Transport is a vanishing cost in digital delivery.

This threat strikes me a very overhyped -- what would stop a competitor from running ads about the "Unlimited Internet" access they offer and decimating the incumbent's DSL installed base?