Idiomatic phrases and constructions are part of linguistic usage and as such not amenable to alteration. A command of one’s own language includes the knowledge of idioms. Violation of the idiomatic norms of a language is a sign of deficiency.
In a recent utterance attributed by the media to Hillary Clinton, Mrs. Clinton mentioned that when she and Bill left the White House, they were “dead broke.” American English does not have such a phrase, the idiom being “flat broke.” One can be “dead drunk” and “dead last,” but not *dead broke (in linguistic notation the asterisk signifies either an incorrect or a reconstructed––hence questionable––form).
How should one evaluate a sin against usage? In the case of a prominent politician like Hillary Clinton (who actually writes remarkably well), one can perhaps chalk the mistake up to the heat of the media moment. At the same time, usage is a form of truth, since by its very fixity, it is immutable. A violation of usage––whatever the circumstances––is, therefore, a transgression against verity, i. e., a sin against truth. Such a mistake, especially emanating from the mouth of a politician, thereby speaks against their veracity.