I went very carefully over the submission guidelines and nowhere is anything said about what rights the author retains. In literary contracts, any rights not specifically claimed by the publisher or would-be publisher are the property of the author. (BTW any "contest" that says it owns all rights to all submissions is not one that I would submit to.)
So in submitting, you still retain your rights.
I was not, however, able to find any rules regarding the winning entry. This tells me that a specific contract will be issued for the property. Given that this is Hearst...I have conflicting thoughts. Hearst is usually pretty good about how they treat writers. On the other hand, Hearst has been known lately for re-using and re-publishing stuff without permission or payment, even when the contract only said "first North American serial rights."
So I would assume that Hearst will present the winner with a contract that basically ties up every single right to that property forever. Just in case, you know, Showtime wanted to make a movie out of it.
So you have to ask yourself, do you want to sell all rights to that story forever for $5000?
Only you can answer that question.