As I observed in an earlier posting, the strictly grammatical way to say those two noun phrases is as follows: "plea of 'guilty'" and "plea of 'innocent'," where the adjectives "guilty" and "innocent" are enclosed by quotation marks to indicate that they are single-word declarative statements rather than adjectives. Obviously, however, those quotation marks are so cumbersome to use for such short phrases, and are also lost in enunciation. We therefore can't blame lawyers for not bothering to indicate the presence of those marks by saying "plea of quote guilty unquote" or "plea of quote innocent unquote." As a result, "plea of guilty" and "plea of innocent" have become stratified in both the official and casual language of lawyers, forming part of their jargon that's known as "legalese."