Not all of them are scams. I've won over 2 dozen national and international contest. Some were free and some had fees up to $50 per picture with really amazing prizes.
You need to do your research prior to entering the contests.
Here are a few tips:
Google those sites and read what past contestants say.
Find the sponsors. Good contests have good sponsors(ex. Canon,Sony,etc)
Find the judges. Most legit contests will tell you who the judges are.
Avoid any contests you find through any spam links(ex. from email spam)
Use common sense!
Always use a separate spam email for anything like that. Avoid any "free" stuff,especially from facebook ads. They are all scams. In fact, half of all the ads on facebook are from scammers. They either want your email address or your credit card information.
Good luck!!