The flesh … shalt thou burn with fire without the camp. Such was the rule with sin-offerings generally (Le Exodus 4:11, Exodus 4:12). The curse of sin which was on them, made them unfit for food and even unworthy of burial within the camp.
On the symbolism of the burial, see Hebrews 13:11-13. His dung. That which the bowels contained at the time of death.