Personal Hotspot and Internet Tethering is an option provided by the carrier. Generally it is a part of your plan however sometimes it won't be and that is entirely up to the carrier. While it is a feature of the phone, it must be enabled in advance before the feature will be displayed.
Sure the error came to be with iOS 6.0, but as with any operating system, the people who provide services that use that operating system must provide the support for it. Think of a video game you might own on Windows XP, then Windows Vista comes out and you install it, you then try to use that same video game however it doesn't work on Vista because of how the software works. Who do you blame? Do you blame Microsoft for releasing a new OS with new features? Or do you blame the video game producer who hasn't yet supplied an update that supports Windows Vista?