It probably doesn't hurt that downloaded material is, by nature, much easier to cache than the uploaded stuff. Also, price discrimination is a factor. Generic home users really don't need that much upstream bandwidth, web browsing, gaming, streaming media, and other such things don't take much upstream. Anyone who does need a nice upstream pipe is likely to be more willing to pay(business website admin) and/or more expensive to the company(pirate king bittorrent user). The ISPs thus want to make sure that such people either pay more.