So how can I get a smartphone if I'm outside of AT&T's market?
Smartphones on other networks are A. phones for which the carrier imposes a similar requirement of a data plan or B. phones comparable in functionality to an AT&T smartphone.
But in your example, why would I buy a phone from AT&T if I'm going to get a GoPhone sim?
To ensure that it works on AT&T's network. (GoPhone is AT&T's prepaid brand.)
Won't I be paying for two phone plans then? AT&T (unused, but still per month costs) and GoWhatever?
Phones for use with GoPhone are sold up front.