This is because they believe in the prohibition of offering slaughtered animals to anyone other than Allah and mention the Name of Allah when slaughtering their animals even if they believe in things which Allah is far above.
Also,it is authentically reported in the Sahih (authentic) Book of Hadith (i.e. AlBukhari and/or Muslim)from `Abdullah ibn Mughaffal (may Allah be pleased with him) that he said: I found a bag containing fat on the Day of Khaybar. I caught hold of it and said: I will not give anything today from it to anybody. I, then, turned around and saw that the Prophet (peace be upon him) was smiling (at my words).
Fuqaha' (Muslim jurists) took this incident as evidence on the permissibility of eating what one needs from the spoils before distributing them. This is the evident meaning of this Hadith. On the other hand, the Fuqaha' of AlHanafiyyah, AlShafi`yyah, and AlHanabilah took it as evidence to refute the opinion of the followers of the Madhhab (School of Jurisprudence) of Malik who consider it impermissible for Muslims to eat the fats and other parts of the slaughtered animals of the Jews which have been rendered unlawful for them to eat.