For rockbox to support USB on the sansa, ipods and a few other players a full blown USB stack has to be implemented in software, this is a lot of work and will take a long time to do if it ever gets done. On iriver h100/h300, gigabeat old archoses, iaudio x5/m5 the usb is handeled by a hardware chip and rockbox just detects the connection and hands over control to the chip.