Can't answer for the OP, but one reason might be the speed rating. The TBolt comes with a Class 4 SD card at 4Mbps. a Class 6 card is 6Mbps and a Class 10 is 10Mbps. Speed can be one reason.
I've read that the TB forces all pictures to be saved on the SD card. This is on area where speed can really matter. Ever wonder why it takes to long after taking one pic before you can take the next? It's because it is writing the last pic to the disk. The faster the disk performs, the faster you can take the next pic.
I do agree however that you'd have to have a pretty specialized need to spend extra bucks to gain performance over the provided Class 4 SD card.
-Frank