If Heartless do not have a state of being sentient, then this question has some merit... not a lot when more information is looked at, but...
Anyways, Hades was a part of the Disney villains who were trying to use the Heartless to their own advantage; Hades is from Olympus Coliseum, so I think it was supposed to just be understood that Hades played a major roll in the Heartless being in the Games in that world.
Even though he used Cloud, specifically, to take out Hercules, the Heartless were completely foreign to the demigod (at least, in KH1), so it makes sense to use them as rank fodder to go against the would-be god (that is, in the Disney mythos). Another explanation for using the Heartless in the Games would be to keep unwanted enemies from getting to Herc, or getting in the way of Hades' plans (considering that it wasn't a death match, only those Hades was controlling would actually kill Herc in the Games).