Because Nichiren teaches that we should never accept offerings from slanderers, not even the offering of the Law:
"Neither Buddhas nor gods would ever accept contributions from those who slander the correct teaching. Then how can we human beings accept them? The deity of Kasuga Shrine proclaimed through an oracle that he would accept nothing from those with impure hearts, though he should have to eat the flames of burning copper; that he would refuse to set foot in their homes, though he should have to sit on red-hot copper. He would rather come down to a miserable hut with weeds choking the passageway, or to a poor thatched house. He declared that he would never visit persons lacking in faith, even if they hung sacred festoons for a thousand days to welcome him, but that he would go to a house where the people have a mind of faith, even though they might be in mourning for a parent. Lamenting that slanderers have overrun this country, the benevolent gods have abandoned it and ascended to heaven. “Those with impure hearts” means those who refuse to embrace the Lotus Sutra, as is stated in the fifth volume of the sutra. If the gods themselves regard alms from slanderers as more abominable than the flames of burning copper, how could we human beings possibly accept them? If someone were to kill our parents and then try to offer us some gift, could we possibly accept it? Not even wise persons or sages can avoid the hell of incessant suffering if they accept offerings from slanderers. Nor should you associate with slanderers, for if you do, you will share the same guilt as they. This you should fear above all.
Shakyamuni Buddha is the father, sovereign, and teacher of all the other Buddhas and all the gods, of the whole assembly of human and heavenly beings, and of all living beings. How could the heavenly gods and benevolent deities rejoice if the Buddha were killed? Today all the people of our country have proved to be enemies of Shakyamuni Buddha, but more than laymen or laywomen, it is the priests with perverse wisdom and hearts who are the Buddha’s worst enemies. There are two kinds of wisdom, correct and perverse. No matter how wise a person may appear, if his assertions are warped you should not listen to him. Nor should you follow priests merely because they are venerable or of high rank. But if a person has the wisdom to know the true meaning of the Lotus Sutra, no matter how lowly he may appear, pay respect to him and make offerings to him as though he were a living Thus Come One . Thus it is written in the sutra. That is why the Great Teacher Dengyō says that the men and women who believe in this sutra, even if they lack knowledge or violate the precepts, should be seated above priests who observe all two hundred and fifty precepts of the Hinayana teachings, and never be seated in a humble position, and that this is all the more true of the priests of this Mahayana sutra."