The answer is because it's at the drive or chipset manufacturer's discretion to include any instructions that are not important for the recording process. Also, reading the firmware date and serial number is defined at a much later date, i.e. 1999, MMC2, which started to define the 'Get configuration' command.
If u look at the specification (well in draft anyway) it does not mention that it will fail if a feature is not supported. It does mention that a check condition status is to be given. But this will not prevent the DeviceIOControl function to fail. Hence why u think it was successful.
One more thing, did u remember to zeroise the data buffer, before u send the command?
The garbage could just be comming from uninitialised memory allocation.
I also tried these commands for myself and none of my drives will return anything:
LiteOn 52327S successfully returned some data, but serial numbers are all spaces, i.e. ASCII code 32
Benq 1620 failed to even return any data.
Unless, I've made a mistake with my source code, I will assume that these commands are not very well supported.