AFAIK the 5.1 audio tracks on the DVD are stored in the raw S/PDIF format already *anyway*
DVD 5.1 audio is most commonly stored in Dolby AC-3, which is a perceptual-encoding compressed multichannel bitstream. There are exceptions -- DTS being the largest -- but, it turns out that no, you can't just pipe the multichannel out an S/PDIF port and get anything coherent unless you've got outboard AC-3 decoding. Of course, you can buy that, but not for $20.
Also, as far as I know, a license from Dolby Labs is needed to get the codecs necessary to uncompress the AC-3 stream into raw audio. Obviously, they've got this license since their other products do have this feature, but I'm sure there is a per-sale cost associated with it. Of course, I doubt that cost justifies anywhere near another $20 retail value.
FWIW, I agree with your other sentiments, but they are justified in deciding how to price their software. It's simply unfortunate that a libre (contrast: gratis) alternative will be run roughshod from the market were it to appear.
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