Should an unlocked phone be able to function on any carrier without hacking? Yes. If I had my way, there would even be laws guaranteeing customers this ability.
However, I really can't sympathize with you at all. You intentionally and knowingly chose to do something that was not officially supported. Now you are complaining that your hack stopped working. If you wanted it to always work, you should not be doing something so weird.
Classic case of "Doctor, it hurts when I do this."
Oops, it looks like you did not read the article. The person in question owns an officially-unlocked iPhone. This involves an anti-competitive practice where the APN settings are locked to a specific carrier, AT&T -- even in the case of an unlocked phone. You really seem to be venturing into an area you're not educated in.
Apreche acknowledged that the settings shouldn't be locked on an unlocked phone, his point seems to be that when you use an "unofficial workaround" as the author claims to have done, you shouldn't be that surprised when it stops working.
It all depends on who the IIN is that issued the SIM card. If it's an AT&T-issued card (which would include all of AT&T's MVNOs, such as Straight Talk), the iPhone just sees an AT&T SIM card, and applies the Apple-signed AT&T carrier profile to it, and in that carrier profile is a bit that controls whether the phone allows access to the Cellular Data Network menu or not, which AT&T has set to "deny."
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.
If the SIMs are indistinguishable, what else could be done (without removing the APN blocking, which is AT&T's will, not Apple's)? A solution for this should be part of MVNO agreements, either providing their own SIMs or dealing with it in their infrastructure.
7. Implement solution while grumbling to self about truth in advertising.
8. It works! (hooray)
9. Software update breaks solution.
I still say the key word here is "unlocked." If Apple truly meant for the "unlocked" iPhone to only be used with supported carriers (and then what's the point? you think Apple's really only selling "unlocked" phones to international travelers?), then they shouldn't be using that word to describe it.
I'm curious. What if you use the iPhone Configuration Utility to generate the output file? I've done this before to roll out multiple phone configurations that all had pre-configured email IMAP and SMTP settings for a large organization.
I didn't alter any cell based network settings. Though I recall the output file from the iPhone Configuration Utility was merely a text file. I believe, though I'm not 100% certain, and this was around iOS3-4, that I opened the file in a text editor. I think I recall this because I came to the conclusion one could make a web based configuration utility that output the same data file.
I'm pretty sure it was just simple XML. It may be a binary plist type file now, but plutil should solve that problem.
Couldn't one manually edit the output file to contain the values they desire, import the file, pop in the SIM, and have a functioning iPhone on iOS6?
Just a thought based on some very old memories. Anyone care to fire up iPhone Configuration Utility and see what file type it generates. Even if not XML, a hex editor should allow the changes, no?. A small git-hub project could probably create all the needed profiles for the various carriers out there for unlocked phones.