The device is bound to cypress manufacturing driver just to program the EEPROM connected to the device with the appropriate configuration information. So the device is not bound to a mass storage driver and hence drive letter does not appear in windows. When the device is bound to mass storage driver, drive letter will appear.
Note:- With AT2/AT2LP series and the older ISD300xx series of mass storage solutions, mass storage driver for earlier OSes of windows are provided. So confusion between mass storage driver and manufacturing driver occurs sometimes.