Cheated gets copied, and in time becomes universal. There was a time when defending a penalty in football was seen as pretty dodgy. Football commentators will regularly talk about someone winning a free kick when a foul by the opponent is seen as having being cleverly provoked.

But I think the line should always be drawn against intentionally dangerous acts, and against actions like Clarke's. Failing to penalise it retrospctively is a way of ensuring that others do it.