The SIM doesn't contain the radios. The radios are on the phone. The low level firmware on the phones determine how the radios operate, the SIM card just provides some additional data to the phone software to help it decide how it communicates with the radios, what systems it is authorized on (software keys), and stores some data potentially. At least thats how I understand it.
The issue is, from what I can tell, that in the US, the radio firmware is highly restrictive to lock in carriers, and not source exists for replacing that firmware with an unlocked/multi-carrier firmware. You need the firmware that enables the use of the radios and phone on a carriers network, the SIM that goes with the carriers network, but also the carriers network allowing your phone onto thier network.
So while there is no technical (electronic or programability) reason for the iPhone 4S or 5, 5S and 5C working on only select networks, it is business reasons (locked in legal restrictions encoded in the firmware and policies governing the carrier networks) that prevent users from being able to use thier hardware on other networks in the US.
This is the kind of stuff we need our legislature to fix; they took a swing at it but its not yet fixed.