I opened up my RT systems cable and found that it has a FTDI chip in it.  It's a standard chip, but they programmed it with their own USB vendor ID codes so that it loads their custom driver.  I doubt they did this to be mean spirited, but who knows for sure.  This gives them the ability to make their software work in a plug-and-play fashion where you don't need to configure any port settings.  Just plug in the USB cable and it will automatically find your radio.

However, because it is a standard chip, I was able to get their cable to work by downloading the regular FTDI Virtual Serial Port (VSP) driver and forcing it to be used instead of the default one.