Well, while it is not the most ethical thing to sell fansubs its up to the sellers to buy them. While you may say that they should download it off the internet, who is to say that someone with a 56k modem has the inclination to expend the necessary time to download 80+ episodes?
Especially in this day and age, time is at a premium - its a sad fact that paying for time has become such a core part of our lives that we dont even notice it.
Ever noticed that things you buy at a petrol station are more expensive than an identical product at a store across the street? Theyre a higher price because people are willing to spend money for the convinence and saved time.