Main objective while playing League of Legends

I have had a lot of fun playing this game. When you have players that are new it doesn't bother me because everyone has to learn. My problem is that there are players that just focus on getting cs and that's it. They don't really want to win or help the team. They just get a ton of cs to be really strong. It has been happening a lot to me lately and it's frustrating to have someone really good not focus on winning the game by teaming up. My biggest issue is that these players are being rewarded with s's cause the more cs the better your grade will be.