It's a precaution, whose source is some guy in legal who wants to cover the company, not based on any logical (chemistry) reasoning. If your FC is lower than shock level for your current CYA, there's little chance for any damage to a properly constructed and designed robot. An FC of 4 with 0 CYA is some pretty harsh water and could be bad for the robot. An FC of 4 with 80 CYA barely has any active chlorine that's not bound to CYA. If you send it for warranty work, tell them the FC was never above 4, even if it was. If they really knew what could be bad for a robot chemistry-wise, they'd be more concerned about pH extremes too.