USB SuperDrive Issue

You can edit a plist file by following the instructions found here to enable it on your system, however be aware you won't be able to boot from it (such as installing an OS via CD/DVD at boot).

I have no idea why Apple place the limitations on things like this, other than the potential that it makes things difficult for people (such as myself) who want to stray from normal configurations by installing two hard drives rather than a hard drive and optical drive in their systems. By making it difficult for the masses, they create a more stable experience overall as they have more control over the default configurations.

It's a combination of the firmware on the device, and the plist file which tells the OS whether to recognise the device. There's something in the EFI which prevents it working at boot level though which I don't know enough about to provide further information on though.

The MBP model you have shipped with version 10.6.3 build 10D2094 -- this is a special build of 10.6.3 with support for that model (and probably some others released at the same time) added. Anything older, including the standard build of 10.6.3, won't have drivers/config info/etc for that model, and hence cannot boot it. It sounds like your retail disk has an older version, so it won't work on that Mac.

If you have the installer DVD that came with the computer, you can use that. Otherwise, if you have another (older) Mac with FireWire, there's a trick you can use: start your computer with the T key held down, and it'll go into target disk mode, which essentially means it turns its brain off and acts as an external FireWire hard disk. Connect it (via FireWire) to an older Mac, start the older Mac from your retail 10.6 DVD, and have it install onto the "external disk" -- the MBP. Then, restart the old Mac from the "external disk" (the MBP should still be in target mode at this point), and update to 10.6.8. Now that the OS on the MBP's disk is new enough to support it, shut everything down (older Mac, then the MBP) and restart everything from its regular disk.