Because APNs don't work that way. There's no "standard" way of doing that. It's not like over-the-air PRL updates on CDMA2K networks. On iPhone specifically, there's only one way of pushing APNs to the phone, and that is if the carrier has a carrier profile. If they wanted to have their own official, Apple-signed carrier profile on iOS, they'd have to play by Apple's rules, which presumably means becoming a contracted iPhone reseller.

Since Straight Talk rides the AT&T network and all of their SIM cards bear the AT&T IIN, there are some additional technical hurdles, some of which are discussed and fleshed out here: http://forums.macrumors.com/showthread.php?p=16266984#post16...

Ideally, APNs would just be another value that the SIM card could carry. But I guess since packetized data over GSM predates the SIM card standard, we're stuck with this need to program them directly into the phone.

This is also a problem with Simple Mobile (a MVNO that I'm trying out with a carrier and development unlocked AT&T Branded HTC Titan). Repeatedly, it will default to T-Mobile or AT&T APN and Cell settings, causing me to lose cell service and internet access. Oddly enough, when they've switched settings and cell/internet is supposedly down, I will get MMS messages. There is no way to set the settings correctly and lock them down from an end user perspective. It's pretty much a wash when it comes to service since there aren't enough T-Mobile towers where I'm at, and after the amount I've already paid for is up, I'm switching to Straight Talk to give them a shot.