By contrast, if every household in a neighborhood turned on all their water taps at once the water pressure would plummet. So too, cellular towers have shared bandwidth (as do their backhaul links) so that the more people draw from them the less there is on a per-user basis. It's easy to provide "unlimited" data when (almost) nobody is using it. However the demand for mobile data is skyrocketing (thanks mainly to the iPhone and iPad) and the current situation is unsustainable.
Cable and DSL net access have the same issue, however because the infrastructure is denser and cheaper, you just don't notice. If it costs a mobile provider a dollar to provide a gigabyte of data that same gigabyte of data can be provided for a penny by the wired infrastructure. The actual numbers aren't important and change rapidly as technology evolves, but wireless data will always cost more than wired data so it makes sense that metering would appear there first.
Strange because Time Warner already tried implementing metered cable usage in a couple cities. Didn't work out to well as I recall.
“...because the infrastructure is denser and cheaper, you just don’t notice...”
This is my point. It is actually cheaper to cover a given area with wireless than it is a physical cable infrastructure. A $100K of equipment will cover 10K homes even in a rural area like Pahrump. That is nothing compared to the cost of actually stringing cable to each home on each street. Cable modem data usage is unlimited and that is who they need to compete with when it comes to users that want unlimited 3G/4G. You say “mobile” but that isn’t really the issue, is it ? Unlimited data is not going to be an issue when people are actually on the move. It is the tethered use while people are at home or at their office that they are afraid needs caps on usage.
“No, the analogy is a good one because mobile bandwidth is a shared resource.”
But that is not why water and electricity are metered. Those are metered because it actually costs money to provide each unit consumed, and this is in addition to the money it costs them to deliver each unit even after the infrastructure is in place. Internet providers only have the infrastructure fixed costs and they don’t increase with the volume of data moving over it. The carriers are not paying for every gig of data moved over their backbones, and they are not actually producing the content so there is no unit cost.
The effort to cap usage is just a way to slow growth so the existing infrastructure will last longer while they make more money back on their capital investment. A flat rate would pay for the water and electric distribution infrastructure because they have fixed initial costs and fixed maintenance costs — just as wireless infrastructure does — the difference is that each unit of water actually costs the provider money to purchase and deliver while gigabytes of data cost the carrier nothing.