They do grow back. I moved a CBS from my wife's tank into my tank and his two big arms got caught in the net and ripped off.
6 days later I was doing a head count and check up on everything in the tank (I do this everyday at lunch) and I was a little concerned because he wasn't looking good and hadn't moved from the spot he was in since being put in the tank. He was still missing his arms.
9 hours later I was feeding the fish and the CBS came out for foos and both of his arms had regrown!
So, at 12:00pm, no arms; 9:00pm, arms fully regrown.
Amazing.