Because AT&T is a corporation, not a non-profit group. Corporations are here to make money. They're not here to make you happy. If they do, it's just incidental.
Anyway, don't you think the data usage of tetherers is going to increase, to the point where it puts a strain on AT&T's network? Don't you think dealing with that strain is going to cost money? Well, AT&T is going to make sure they charge you for that. The question is, how much?
Since I don't take my notebook with me everywhere I go, I don't need to make sure that I always have a connection for my computer. That being said, it'd be nice if they offered something similar to the iPad's plan. One would be a limited amount of data for a relatively low price and an all you can eat plan. If I don't have to sign up for a separate contract, I'd be willing to go official whenever I do need the tethering.