are fantasy sports sites that charge you to play and give a monetary prize to the winner considered betting sites?

gambling sites such as sites were you can play poker are illegal in the US right? But what about Fantasy sports sites that charge you and give money to the winners? It seems like you would be betting on your team to win in a way and would then be illegal. But on the other hand your just paying to use the site with the potential to win something at the end, not necessarily through a bet.