But tethering costs $20/month and you don’t get any extra bandwidth at all. If you don’t get extra bandwidth, what are you paying for? It’s one thing to charge extra for tethering on an “unlimited” data plan, but it’s outrageous to charge $20 when the bandwidth is already capped.

Essentially, you don't get a bucket of extra data, but you get to pay $20 per month for the "privilege" of connecting another device to use the data you already have. This strikes me a bit like the power company charging you for the electricity you use and every lamp you have in your home.