I've seen many cable set ups that are grounded to the sprinkler system like that. I've seen them where it connects to the copper at the BFV, but it is a poly mainline in the ground and that blue PEX(?) inside the house. So not sure what the heck good it was doing.
I've taken them off the BFV when I needed to replace them, and never hooked it back up. Keep your paws off my system.
A lot of them are grounded to the outdoor hose spigot. Once again though, if it is the PEX inside, no help, right? I'm no grounding expert nor do I play one on TV.