Hi. This is by design, Waffle. Even with CD's, it may still leave the "max" burning speed option, however, if the disc is rated at 16x, "Max" stands for 16x in this scenario. all media, including CD's, do have manufacturers ID's, that tell the drives firmware what it can be burned at. You can get modified (or hacked) firmware, that will allow you to burn at 12x, and in some cases, 16x, however, on 8x media.
You should download, and install "DVD Decrypter", as this will allow you to see the manufacturers ID, and the speeds which you may burn this media at using your current firmware. (As well as being a superior program, freeware, for burning .iso's, and .nrg's - as well as ripping copyright protected DVD's for personal backup.)