I have looked in Mefarshim and have not yet found anyone who asks your question. I asked a leading Talmid Chacham here and he thought about the question quite a bit and commented that he is not sure yet that this is really a question because it seems that we never found an example where the Torah prohibited benefit from an item for one person and permitted it for the rest of the world. However he cited a few sources to tackle this subject.

(1) My initial reaction to this question was that if indeed only the owner is prohibited to derive benefit from the ox, then this must mean that the Din that the ox must be stoned, is a punishment or fine on the owners. However this latter question is actually a dipute between Rashi and Tosfos in Bava Kama 41b Tosfos DH Al where Tosfos cites Rashi Zevachim 71a DH Al who writes that if the owners admit that their ox killed, the ox is not stoned on the basis of their testimony because of the rule: "Modeh b'Knas Potur" - if one admits to performing a transgession for which the punishment is a fine, one is exempt from the fine. One learns that Rashi maintained that killing the ox is a fine on the owners as a punishment for not having looked properly after their ox.

(2) However the above Tosfos asks on Rashi that we have found nowhere that the fact that the ox is killed, is considered a fine. This is also the opinion of the Ramban on Chumash Bereishis 9:5 that the stoning of the ox is not a monetary punishment for the owners, and the proof for this is the fact that even a desert ox is stoned (Bava Kama 44b) despite the fact that it has no owners. (See also Rashah Bava Kama 45b).

(3) So according to Tosfos one can answer your question by saying that since killing the ox is not a punishment for the owners, it follows that there is no reason to differentiate between the owners and everyoe else, so once the verse tells us that the owners must not derive benefit from the ox, it automatically follows that it is also forbidden for eveyone.

(4) Howver your question still stands according to Rashi. In addition Ritva Kidushin 56b appears to agree with Rashi, because he writes that when the Torah writes that the ox must be stoned, this is a fine for the owners.

Therefore to answer your question we will have to try something else. This is based on Teshuvas Noda b'Yehudah Orach Chaim 1:20 who discusses a case where someone passed away after midday on Erev Pesach, and left a coniderable legacy of Chametz. The problem was that he had not sold his chametz or performed bitul chametz before he died. The question now is are the heirs allowed to derive benefit from this chametz? Noda b'Yehuda dicusses the question at length and in the middle of the Teshuva (DH v'Hineh Makor) he says an intersting svara. For chametz that was not sold on Pesach, Chazal fined the owner and prohibited him from deriving any benefit from it even after Pesach. Noda b'Yehuda discusses what the Din is if the owner died - did Chazal also fine the son? Noda b'Yehuda writes that even if one would say that they did not fine the son, this however would only apply after the father had died, whilst as long as the father is still alive, one certainly must say that the son is also forbidden to derive benefit from the cahmetz. Noda b'Yehuda writes that this is because as long as the father is alive, since the father is forbiden to derive benefit from the chametz it must be that everyone else in the world is also foridden to derive benefit from it because if it would be permitted to others this would automatically mean that the father also enjoys the chametz since he could give it to others as a present or feed it to the dogs.

(5) One learns from Noda b'Yehuda that it is impossible for an item to be forbidden for one person to derive from, but permitted for the rest of the world. So now we have an answer to your question:- since Ben Zoma says that the owners may not derive benefit from the ox, it follows that the ox is forbidden to the whole world.

D. Bloom