Because if they don't, "there customers" will suffer. If their customers suffer, they will switch to better ISPs. If they switch to better ISPs, they won't make as much money in the long run.
If your thinking worked, we'd all still be using dial-up because it's more profitable for the company to operate a simple service and charge loads in the short run than to improve and expand and offer a better product. At the end of the day, another ISP will always be seeking to gain a competitive edge by providing a better service, so I'm not worried about my own downloads. I've already said I am probably not even a heavy user, but I would much prefer to choose 1 ISP and stick with them for many years, than being branded a "leeching *******" as soon as the "regular" user catches up to the idea of the "internets" and having to switch.