The 5.1 option is only available, if 5.1 speakers are connected and the decoding has to be done by the onboard audio. If you connect the optical, the decoding is done by your receiver and the option is unavailable.
Sounds normal. Most motherboards, while "supporting" Dolby Digital Live and DTS-Connect, don't enable the feature due to licensing fees. As a result, you are limited to stereo when using SPDIF, except in the case where audio is already encoded as either Dolby Digital or DTS.
Most receivers do have Dolby Pro Logic II support though, so you can fake it if need be. Pro Logic doesn't do the best job however.