Previously AT&T charged 30.00 a month for an 'unlimited' data plan. In reality all of these plans have/had 'soft' limits of around 5g if you read the fine print, but I'm not sure it was widely enforced.
AT&T offered tethering plans for most of it's smart phones, excluding iPhone for some reason, for around another 30.00 a month. People complained that it wasn't fair to charge you more money just to access the data you already purchased from another device. The example of your home internet was used. You pay for the pipe to your house, and you can hang as many devices on it as you want. AT&T's response was that tethering to a computer would actually increase the amount of average data you consumed on your plan and they needed to cover those costs.
AT&T promised that tethering was coming to the iPhone almost exactly 1 year ago. It still has not.