Sorry, I have no code reference as I don't know of it being an NEC issue myself. My son is a technician for Comcast and I ran this by him, although he referenced 820-41 and 250-71(b). Now that I've looked, neither of those articles appear to address this question specifically in my mind.
I really can't say for sure except that he said that they are trying to avoid the presence of ground loops and the inevitable non-metallic plumbing isolation being carried by the cable shield. I guess that part would make sense from a precautionary standpoint.
