er.. it's more than just "we don't want people to host sites". That's a bit of it, but for cable systems there is a technical reason for it.
I'm pulling this from memory, and the speeds have increased since w/ the 2.0 specification, but in general the percentages should be similar...
DOCSIS 1.0, e.g. what cable systems use to communicate specify the upload channel of a cable segment (read neighborhood that is sharing the copper plant) of around 2.1Mbit/sec iiirc. This means 2.1Mbit/sec for everyone on that segment, which can be thousands of individual modems. The downstream channel is considerably larger, around the size of a ds3.. something like 54Mbit/sec.
Feel free to look up the current specification now, but you'll see why there are such restrictive upload caps on cable plans.
For ADSL it's again largely the same thing, a portion of the spectrum is carved out for upstream communication, and a much larger portion for downstream. Newer DSL tech obviously is more symmetrical however.
So yes, there is the "don't host stuff!" business case, but in general ISP's have tons of "free" outbound capacity and are not close to using it yet, but the last-mile tech simply isn't there right now. Now, once FTTH gains significant penetration there really are no more technological excuses left
