5.56 vs .223 and reloading dies

[bluebayou]

This is more of a theoretical, philosophical kind of question and might be off topic, but I do have a .223 Handi and a warning from NEF not to use 5.56 ammo so...

The debate between whether 5.56 and .223 are the same cartridge raises its head all of the time on other forums.  I was thinking about buying an AR15 at some point and noticed that you can buy NATO chambers or .223 Remington chambers.  I know that the leades/throats are different.  Fine.  I also know that NEF doesn't want you to use NATO ammo, probably for liability reasons.  

So my question is if 5.56 and .223 are NOT the same cartridge then why can't I buy 5.56 dies from any of the major die makers?  Do all of those 5.56 AR15 people just shoot surplus ammo?  Furthermore, with the long throat that my Handi has then what HONESTLY prevents me from shooting NATO ammo (as long as it stabilizes in the 1 in 12 barrel)?

[handirifle]

The dimensions are the same, thus the reason for only one die.  The pressures are different (higher in the 5.56) thus the differences in chambering.  The 5.56, in my understanding, is chambered longer for the pressure relief.