SlashDot: "Comcast Gunning for NAT Users"

The point is that customers don't pay for 100% of the available bandwidth.
Customers couldn't afford to pay for guaranteed 100% BW to all desinations
all the time. Hence, companies determine how much BW a typical user
is likely to use, build to that, and charge the customers based on how
much it cost to provide it. When folks use the service atypically, they are
using resources they didn't pay for.

The company I currently work for pays has a contract that causes us to pay around $50 / meg. A typical cable customer (by our traffic to customer count) uses about 5K, on average. If we are paying $50 per meg, should we be charging this customer $.25 a month for Internet transit? Granted, we have power and Juniper routers, and OC48s to pay for, but that also should be very small.

I think that the cable companies are more concerned with you stealing IP address space, and possible denying service to another customer because of it.