Most designs have a CAN transceiver that takes TTL/UART and implements the differential signalling used on the CAN bus. I've used the TJA1048 before in designs

Most designs have a CAN transceiver that takes TTL/UART and implements the differential signalling used on the CAN bus. I've used the TJA1048 before in designs: http://www.nxp.com/documents/data_sheet/TJA1048.pdf http://www.nxp.com/documents/data_sheet/TJA1048.pdf You could certainly clip the TX line and make it receive-only if you wanted. But there's always one person in the room that says "hey, why not leave it in in case we change our mind about software?", or perhaps it's more "hey, let's copy the reference design and move on".