I'm a network admin, so I know all about running out of IPv4 addresses. What I was talking about is if you look at the user agreements from some ISPs, they will say something like you can only have 1 computer on their service. AT&T DSL, for example, used to have a extra paid "service" that allowed you to network your home.
I definitely recall this in detail, as I've worked for AT&T as an engineer (mostly contract design and analysis work). But as I said, they didn't actually have (nor do they at this time), have the infrastructure to attempt to force users to have one line per system. 2x lines is the max possible (physical cable dropped to most homes, not businesses, which if in the right location, have access to other signals, such as Tx, Sonet,...), and in some areas, they can't even provide that (AT&T has 3x zones, A, B, and C, C being the lowest priority). For C zones, most are maxed out @ 1 DSL signal per address.
And it's from the additional fee you mention that they're well aware of consumer backlash.
A router not only solves the issue for consumers (save bandwidth limitations), but it saves the necessitation of upgrading the infrastructure to meet a billing policy (expensive, and why they like to coincide it with government projects; 1. they can get the gov. <fed, state, and on occasion, local> to help fund the upgrades due to forced moving of physical cables, but 2. it's also far easier in terms of permits).
Ultimately, they had to compromise on their greed (more $$$ per physical location in cases of multiple systems within it).
They would say only one computer per service, but you could get around that by connecting a router to the ISP modem. That way, the ISP only sees one MAC address and is happy.
Given the physical capabilities of the infrastructure, they're willing to allow for a router and not complain for consumers. For businesses however, it can be a bit of a different story, depending on both size, and available services (DSL, Tx, Sonet).
Most cable providers only allow one MAC address to connect to their service, which is why you need to hang your stuff behind their router. Basically, though, if their equipment is a combo cable modem/router, I don't see a problem sticking with it for most people.
I've not worked with Cable providers directly, but wouldn't be surprised at all that they're in the same situation as the DSL providers in terms of infrastructure issues. And again, greed has resulted in the single MAC address per line, but will allow a router to be used without issue (i.e. I am familiar with Comcast providing modem + router devices at the location). For other cable based providers, I can't be absolutely certain, but wouldn't be surprised if they did the same (would make sense anyway).