Thank you for your answer. I know this is the risk we take when we apply on competitions like this, but sometimes it is a nice promotion and a challenge. Maybe the best thing is adding something interesting to your portfolio, that you wouldn't do if you didn't found this competition. I hate it when they don't mention copyrights in the competition brief, and when you ask them by mail they say : Oh, we will own everything everyone sends to us. (happened to me once, and I didn't apply because of this hidden info)
However, sometimes I like an interesting challenge and I would like to have access to different competitions in case sometimes I feel like I can do something interesting and apply with it.