Some sort of AT&T's policy. To enable tethering, it used to be a single MobilConfig installation until Apple required them to be signed on 3.1.2, presumably to

Some sort of AT&T's policy. To enable tethering, it used to be a single MobilConfig installation until Apple required them to be signed on 3.1.2, presumably to enforce AT&T's policy, which also result in tethering being unusable outside any "Apple-certified carriers" even in the unlocked countries.