Lotteries are illegal unless officially licesned by the state to do so), contests are a different story, but you need to research all 50 states to know the do's an don't if you open it up across the contry. I know there are 7-8 states where an internet contest's are illegal or highly restricted. There is also a difference between a game of skill vs a game of chance.
If you read TOS os different contests, you will see this. The reason I know this that I live in one of the states where I am illegible for some contests (Maryland).