While some carriers in some countries allow for free tethering on the more expensive data plans, many countries, including the US do not. That means those carriers make extra money charging for tethering plans, and apps like Netshare and iTether threaten that extra money (or double-dipping, as you may wish to call it.)
I don't know if Apple has carrier agreements in place to prevent tethering apps and protect revenue streams, the way they once did to prevent VoIP over 3G, but should that really be the users problem? Should it be Apple's to police? If you get a tethering app and stay beneath your data cap, shouldn't that be between you and your carrier? And if you're on an unlimited plan, aren't they already sending nastygrams if you go over a certain amount anyway?
TiPb Nation, what do you think? Should Apple allow tethering apps in the App Store?