One of Rabbi Yishmael's disciples raised a distinction between different categories of oxen. An ox that has become ritually impure (tamei) is still permitted for deriving benefit — you cannot eat it, but you can use its hide or other parts. Since benefit is allowed from a tamei ox, its hide is permitted even in death.
This contrasts sharply with the stoned ox. A stoned ox is forbidden for all benefit — not just eating. Therefore, its hide should be forbidden in death as well.
The distinction matters because it prevents a false equivalence. One might argue that since the tamei ox and the stoned ox are both dead animals whose flesh cannot be eaten, they should be treated identically regarding their hides and other non-food products. The disciple rejected this reasoning.
The tamei ox is forbidden for eating due to ritual impurity, but benefit from it is still permitted — so its hide remains usable. The stoned ox is forbidden for all benefit due to its legal condemnation as a killer — so its hide is forbidden too. The source of the prohibition determines its scope. Ritual impurity restricts only consumption. Legal condemnation restricts everything. Two dead animals that look identical on the surface are treated entirely differently by Jewish law because the reasons for their respective prohibitions are fundamentally different.