Why is it canonical?

An example of free organum, from the Ad Organum Faciendum treatise. Free organum, "[utilizes] parallel motion and oblique motion (upper voice moving while the tenor holds one note), but the introduction of contrary motion (voices moving in opposite directions) as well as similar motion (voices moving in the same direction, but to different intervals) led to progressively freer musical lines—a prerequisite element of counterpoint." [Organum, Wikipedia]