Ahh but they are giving you something in return.I think it should be illegal for AT&T to charge for something without offering anything in return, isn't that the legal definition of fraud??? /s
The ability to tether.
The claim that your unlimited plan means you can use it however you want is completely false.
Usage is subject to terms and conditions of the contract.
Tethering is not included in those terms.
Like it or not, it is a contract violation (not illegal) to use your data plan via tethering.
Hence the add-on charge.