I think the suggestion is more along the lines of you buy a set bandwidth with a certain contention ratio (or ratios) and then the ISP lets you contend for the bandwidth as you like.

Having a bandwidth cap is basically the ISP punishing you for contending too often.

That said, assuming the ISP wants to keep an average speed higher than the minimum their contention ratio would offer, people who download a lot would have a greater impact on that, and could therefore cause the ISP to have to invest in more hardware and back-haul.

The counter is that the ISP isn't upgrading their network anyway, they're just juicing the market by charging some people more.