The reason why so many people who criticize the immorality of contemporary society are then found to engage in these same activities is due to the defense mechanism of

Defense mechanism of repression.

Explanation:

Repression is a defense mechanism that helps a person keep distance from his own desires because of guilt. A person who criticize the imorality of certain behaviors or actios, and is later engaged in the same activities is because the person repressed wanting to do it.

Repression occurs in order to avoid guilt. Repression allows thoughts to stay unconscious so a person doesn´t have to deal with them. Normally repression happends because a person cannot accept his own dessire, because sometimes it contradicts with his ideal self.