Constant bitrate on the CA modules is used to ensure a constant rate to the CA module. This has to do with timing inside the CA modules.
Some CA modules have a hard time coping with an incoming bandwidth that fluctuates too much. If the card speed is set to 50 Mbit/s then the FPGA adds Null packets until the accumulated incoming bandwidth is 50Mbit/s.
FE ---> FPGA add stuffing ---> CA descramble ---> FPGA remove stuffing ---> BE
As you can see above, the stuffing is removed again, after descrambling. All this is done, on the BE hosting the CA module.
This is also the case if you use transparent mode.