"All the True Word priests go through a secret ceremony of anointment, in which pictures of Shakyamuni Buddha and others are painted on an eight-petaled lotus and the participants tread on them with their feet. And because those who took part in this bizarre ritual were revered and treated as the supervisors of various temples by the Retired Emperor of Oki, power passed into the hands of his common subjects, and he met with disgrace in this life." -- Repaying Debts of Gratitude
And they hate Shakyamuni Buddha for the same reason as the Shingon adherents; jealousy. There is no comparison between Shakyamuni Buddha and their principle Buddhas (Vairochaina for Shingon and Daisaku Ikeda for Soka Gakkai). Shakyamuni Buddha so outshines Vairochaina Buddha and Daisaku Ikeda that they hate Shakyamuni Buddha for it.