And I also have the question of "why" would fusion automatically mount a CD on insert, when it ask where you want mount a USB drive (it seems like a bug or a very misguided "feature".)
USB Device and the CD/DVD are handled differently. If the Virtual Machine's CD/DVD is connected then it will automatically grab the Disc when inserted on the Host and no arbitration is necessary as with USB Devices and therefore no prompting. While I mainly use ISO Images nonetheless, as a general rule, I typically keep the CD/DVD disconnected in the VM unless I specifically need it in the VM.