Which is another thing I'm curious about... How does their agreement with AT&T help the end-user experience?

It seems there are plenty of people who would be happy to use another carrier. And, in other countries, you can apparently buy unlocked phones from Apple. So the agreement with AT&T seems to be more about making money than about making great products...

Probably a combination of that and three other important things:

a) compromise on things like this in order to gain bigger freedoms, namely, the App Store - carriers are usually getting a cut, but not in this case

b) poor consumer protection in the US - carrier unlocking is, at least in some countries, required by law

c) (related to above) young, immature mobile market in the US

That's a good point, but if it's the case, Apple was not forthcoming about that. I believe the statement said something about "duplicated functionality."