Nichiren said that the reason for the criticism was due to the Zen sect not being based on Buddhist scriptures and having the principles of kyouge-betsuden and furyu-monji (both meaning that the principles are inside one's heart, not on scriptures), but it is believed that his criticism was on the aspect of a person possibly falling into yakozen.
その理由を教外別伝・不立文字で経典に依らないからとしているが、おそらく野狐禅に陥るという一面を取り上げて批判したものと考えられる。