It looks like Apple doesn’t want people on unauthorized carriers to share their mobile internet with others.
When Apple first launched the Personal Hotspot feature it received much criticism from carriers. The feature lets iPhone users share mobile internet by creating a standalone network.
That changes with iOS 7.1. Users who have updated to the latest firmware report that they can no longer manually save APN settings for Personal Hotspot. They can still add settings, but as soon as they back out of the menu in the Settings app, whatever they entered simply vanishes. This was initially believed to be a software glitch, but there’s evidence to support that Apple might be cracking down on people abusing the feature.
There’s a 15-page thread on Apple’s official support forums about this issue and countless users have reported facing the exact same issue on their iPhones after having updated to iOS 7.1. It comes down to whether or not a mobile carrier supports Personal Hotspot. Users on carriers that support the feature report no problems with using Personal Hotspot even after updating to iOS 7.1. Moreover, users that switched from unauthorized to authorized carriers after updating also faced no problems in using the feature. This goes to show that it is not a software glitch, in fact, its Apple ensuring that there’s no abuse of the feature on unauthorized carriers.
In the past, many third-party applications have also surfaced, allowing iPhone users to share their mobile internet. Apple has always dropped the banhammer on them as it’s staunchly opposed to unauthorized tethering apps.