Defenders of Metered Broadband argue that other Utilities such as Electricity and Water are all charged based on usage: The more Watts-hour you consume the more you pay. So why not the same for broadband: the more gigabytes you download, the more you pay. Yet there is a big difference. Water and electricity are limited scarce resources, but bits are not. The more electricity we consume, the more that needs to be generated and that has a cost (as Al Gore taught us, not only for our pockets but also for the planet). In Broadband, once the bandwidth is provisioned, the transferred bits have a zero marginal cost for the ISP. The capacity has the same cost whether idle or in use.
Broadband services should instead be compared with Pay-TV services. The marginal cost of people watching one more hour of TV is zero. That is why no one could imagine that a Broadcaster or Cable Operator would cap the number of hours of TV that you consume. Imagine Comcast or Canal+ limiting to 4 hours per day the Pay-TV you can watch. That would be good for our mental heath but insane from the user point of view. What Pay-TV Service Providers limit is the capacity offered: e.g. basic package has 40 channels and you can upgrade to premium packages with additional channels. Similarly, ISPs are expected to charge based on capacity (bandwidth) provided, with premium options (more bandwidth). But please not on usage.
Note: Only Mobile Operators can rightly argue that spectrum is an scarce resource, and therefore they need to cap traffic to preserve it.