Dear internet people with concerns over the excluded countries.
Please search for "Office of Foreign Assets Control (OFAC)." There are other countries on the sanctions list than the ones excluded here, but it's probably an issue of not being able to send the winner the prize money if they live in one of those countries due to federal law.
I'd say it's preferable to know you're not elgible up front than to win and get the "Thanks for the great photo, but sorry we're not allowed to give you the prize because of where you live." email.
That would REALLY stoke the "this is just a grab for free images for commercial use" conspiracy.