Not exactly. The bulk of the construction costs are at the last mile. Wires into homes are built regardless of whether a homeowner uses them, and neighborhood-level equipment must be built and operated (at roughly constant cost per house) regardless of how much downloading or uploading occurs. So it doesn't automatically make sense to count local communication toward a bandwidth cap. Instead, the standard monthly fee seems like a fairer way to cover those costs (and the fee is certainly high enough to do so!)
Long-distance links, while also a fixed cost to build, are used at capacity or near capacity; therefore, they tend to be built according to demand and so their cost is dominated by the amount of bandwidth needed instead of by the number of potential customers.
It does, therefore, make some sense for Comcast to "forgive" local traffic since it usually doesn't go though very many saturated internet links. To evaluate fairness, my question would be, is this a special exemption for Xfinity, or is all local traffic (e.g. file sharing, Skype) forgiven, within, say, city limits?
Of course, if the long-distance costs are very small, as you imply, then that just means the long-distance bandwidth cap ought be large, and overage fees small.