It was a pretty daft rule in the first place. Footballers are only human, and also bear grudges. I can remember back to when the likes of Billy Bremner and Nobby Styles would probably have preferred to throttle each other rather than shake hands. You can also count others in that same category like Jackie Charlton, Alan Ball, Dave MacKay, Norman Hunter, Ron Harris etc etc.
Some of these guys would have stuck the head on one another instead of having a poofy handshake!