This application is really for the following:
1 - US users who are grandfathered on AT&T's Unlimited data plan
2 - Other users who don't have the Personal Hotspot option
FYI - I live in Denmark and purchased my iPhone in Europe. Out of the box, it was unlocked. The carrier in Denmark allows my phone to 'Teather' with a personal hotspot.
When I travel to the US (Which I do every month), I use my AT&T sim card in my iPhone. I have had an AT&T account for 10 years or so, (Cingular et. al) and was fortunate to have the Unlimited data plan. When I swap my sim cards from my Danish Sim to my US sim, the 'Personal Hotspot' option disappears from my Network Options. Therefore, it is the carrier that determines the functionality, not Apple.
my 2 cents.