Two possibilities for this, Ben. (1) They didn't feel market demand would be high enough for shooting RAW to bother modifying DPP to support it; i.e., they didn't want to devote engineering and testing resources from the DPP team to support the G9. I find this unlikely since the G9 raw format is a CR2 and internally quite similar to ones from recent DSLRs. Or, (2) they didn't want users to be able to disable certain aspects of raw image processing, such as noise reduction, sharpening, etc.
Canon is not the only one to do this, BTW. Nikon's recent P6000 is interesting because it has a NRW file format, which ... err, is actually an uncompressed NEF inside (so the raw files are actually a whopping 20+ MB for a 13.4 MP camera compared to ~15 MB for a 14.6 MP lossless compressed raw file from a G10). But you can't use Capture NX to process the files. You can use View NX, but not to adjust any settings (e.g., you can't adjust noise reduction).
I am guessing that Canon feels comfortable making DPP available for the G10 because they are more comfortable with its raw image quality compared to the G9.