It is our assumption of others that they also are committed to the principles of truth, normative rightness and sincerity that allows us to lie - if those who listened to us did not believe we were telling the truth it would be impossible to deceive them.

Manipulative speech does not go against the rules of honesty (which Grice observed and categorised as his Maxims), it flows with it and uses the inherent flaw, the assumption they are hearing the truth, of the participants in the conversation in order to manipulate it.