An unlocked CMDA phone can be used with a any CMDA carrier so long as the frequencies the phone work on are correct, and the network codes are set. You can in fact use your locked CMDA phone in Taiwan, but you will pay about $100 a min to Sprint, or Verizon.
As for why they lock phones, true it relates almost %90 to GSM. T-Mobile wants you to pay their inflated roaming fees when you travel. They do not want you to buy a SIM at 7-11 during your vacation in Taiwan. In almost every country SIMs can be purchased just about anywhere. That is why the law was lobbied in.
Almost all modern GSM phones, and about half of all CDMA phones work internationally assuming the country has the appropriate network in place. for example, my old Verizon phone worked just fine in Taiwan.
Sorry about the soapbox but governments and I have never seen eye to eye on laws. Once you travel you see just how unjust laws are passed to favor corporations, and generate revenue. For example, why is Aspirin illegal in some countries? Simple, because it is too cheap. It competes with local brand name over the counter meds that cost much more. The law is lobbied in to make sure most everyone buys the ten times more expensive band name drug.