Star Trek is the property of CBS. Even if that wasn't the case, Rod Roddenberry represents Gene Roddenberry. He's not in the writer's room, but he's complicit with what's being done on Discovery. If Rod feels strongly against what's being done, he can speak up or -- at the very least -- disassociate himself from the series.
Gene Roddenberry, for all his complaints, never walked away from the Executive Consultant title for the TOS Movies because he liked the paycheck that came with that position, even if his say meant next to nothing. So he didn't believe in his objections strongly enough that he'd refuse any type of credit besides "Based on Star Trek created by Gene Roddenberry".