Not always true. I know of markets in Texas and New Mexico where the A and B carriers are both CDMA.
Nonetheless, carriers nowadays have "preferred roaming agreements" with various carriers in different regions, and often prefer that if you roam, you roam on those carriers. In the case of CDMA, switching to the right band is governed by a "preferred roaming list" (PRL) that is loaded into the phone's firmware. If the phone can't find its home SID, it will scan for other available SIDS based on a "priority list" and lock onto the first one it finds. This ensures that the carrier gets the "cheapest rate" for roaming. GSM has something similar (I forget the name of it), as does Nextel (called a "band map").
Of course, this takes the ability to chose a roaming carrier out of the hands of the end user, but for that sacrifice, you get expanded coverage at "home" rates. None of the large wireless carriers cover every inch of the nationwide network they advertise. A lot of holes are filled by these preferred roaming partners who agree to charge the carrier a discounted rate, who then in turn bills you as if you were on the "home" network. In exchange, the roaming partners are guaranteed a revenue stream from all users of the large wireless carrier that pass through their territory. That's where the PRL comes in (and why end users lost tha ability of manual control).
E-mail fudged to thwart spammers. Transpose the c's and a's in my e-mail address to reply.