The thread where you posted in my Spirit2 forum may be the best place to discuss this:
http://xdaforums.com/apps/spirit-fm/q-fm-transmitter-app-z-ultra-t3056747
Transmit capability in Sony Qualcomm FM/combo chip devices may be an "accident".
To transmit, the so called "Internal Antenna" pin of the chip must be connected to wired headset as the antenna. Most other Qualcomm chipped devices use the "External Antenna" pin which can only receive.
I'm not sure why Sony used the "Internal Antenna" pin. Hypothetically, the electrical characteristics, like impedance, of that ping may have suited the designers better.
But I lean towards the idea that designers wanted to be able to provide an FM transmit ability, while management did not want the extra costs associated with app creation and testing for this.
T-Mobile did this with a phone once; I have a leaked transmitter app and it works on 2 of my now very old phones. They changed their minds.
If nothing else, you can get a phone to market faster without additional testing, and the smartphone market has been fiercely competitive to get their devices out first.