Ethics of Illegal Tethering

First off, tethering is not "illegal". It is NOT a criminal infraction nor a misdemeanor. It is simply a violation of your contract or TOS. The only exception to this is if you have hacked into a carriers network and are receiving service for free. What you do with your paid for data is your business and only your business unless it's related to criminal activities.

Addendum:
The only reason carriers ban tethering is to generate additional revenue by "sticking" it to you to buy an aircard/USB dongle. If you have a capped data plan, how you get it is irrelevant. And the same goes for unlimited plans.