It seemed to be a common kind of agreement, but that doesn't mean it's any good. It's unlikely that anybody will be exploiting the contest artwork, but the language fully allows that. What if some beginning artist becomes famous in the future either for their art or some other creative endeavor? They could suddenly see their contest entries being trotted out for various promotions that they have no control over. This would be something akin to models having to deal with nude photos shot early in their career resurfacing once they're famous.