Why couldn't they eat torn beasts?

"IN view of the detestation in which death was legally held by the entire institution of the law of Moses, it is not wonderful that the Israelites should have been forbidden to eat “that which died of itself or that which was torn with beasts” (Lev. 17:15), or that the same imputation of uncleanness should arise in such a case, and the same necessity exist for purification. It was contact with death in a more intimate form than by touching a dead body or entering a death-defiled tent. It might be supposed that eating flesh-meat in any case would be the contracting of this defilement, seeing that creatures must be dead before they can be eaten. It would have been so if the law of Moses had been a merely hygienic system like vegetarianism, or any other attempt to found human feeding on the natural effects of certain foods on the human system. But the law of Moses was not a hygienic system, though all its principles were in harmony with the best hygienic principles: it was a system of spiritual significances adapted to serve the double purpose of physical well-being and spiritual education. Therefore, while forbidding the eating of the flesh of animals that had died a natural death or been slain by other animals, it could consistently allow the eating of flesh properly killed; because although the physical state of the flesh might be the same in both cases, the allegorical bearings were not the same. Flesh dying of itself would be flesh diseased, and flesh rent for the sustenance of beasts of prey would be flesh dying in animal wantonness or in accident—neither of which could prefigure the sinless Lamb of God laying down his life in obedience to the commandment of the Father. So far as physical considerations were concerned, the meat in question was fit enough to be eaten. Hence, the Israelites were at liberty to “give it unto the stranger that is in thy gates that he may eat it: or sell it unto an alien” (Deut. 14:21). As for themselves, they were “an holy people unto the Lord thy God,” and therefore bound by all that was involved in the law given to them.

1897 Christadelphian p431 (Law of Moses)