That is because GSM operates simultaneously on 2 different frequencies. This is why you can use 2G for voice and 3G for data, aka simultaneous voice/data. This is also why GSM 3G typically have lower battery life on average than their CDMA counterparts.
CDMA operates on 1 frequency at a time, the entire CDMA network is 3G and I dont believe you can turn off the setting, it's more of a fall back mode.