One problem is the change in attitude towards the laws of the game (and this applies to all codes). Originally it was assumed that everybody knew how to play the game, and laws were only introduced where there was any disagreement - I seem to remember that Rugby School had a law stating that a boy sitting on a certain branch of a certain tree was "off his side". As the various codes developed and began developing 'game plans' and being coached, there developed a tendency to assume that if something wasn't specifically prohibited then it was a legitimate tactic. Players are also expected to push tje referee's interpretation to the limit, hence the other Shankly quote.