1UP contacted Red Octane for comment and they replied: "Our products require considerable time for certification and testing to perfect both our software & hardware, so this was the most effective way for us to bring a high quality wireless solution for PS3 this holiday." Since that didn't adequately answer why, they pried further, since the Wii uses Bluetooth but won't require such a crazy USB device. Apparently the Bluetooth technology only works with the Sixaxis and Sony doesn't have the technology set up to allow third-parties to use it. That's a little strange sounding.
Sony gave vague answers, using the word consider many, many times: "Many factors were in play regarding the decision to go USB wireless vs Bluetooth for GH3. However, we are considering Bluetooth for future consideration." Everything else is classified! This is a minor issue, but speculation seems to surround the PS3's strange de-sync with the Sixaxis. Surely that would be a horrifying situation while strumming your axe, so perhaps Red Octane decided not to risk that problem on the PS3. Who knows. Does anyone really care? Wireless is wireless, right?