If you put in a SIM card that the iPhone has no clue about (such as T-Mobile U.S.)., then iOS will fall back to a generic carrier profile, which does allow access to the APN editing submenu.
The problem is that Straight Talk is an AT&T MVNO, and so Straight Talk SIMs are indistinguishable from AT&T SIM cards from the iPhone's perspective. The iPhone engineers made a bad assumption, which is that all cards issued by carrier X should be treated exactly the same.
...I still think APN blocking is entirely irrational on the part of the carrier. There's no way a user can subject their carrier's network to fraud or abuse by having access to the APNs (http://forums.macrumors.com/showthread.php?p=16266682#post16...)