No, your contest will draw attention to Contestors. While a portion of your entrants will be Contestors, a portion of the additional traffic they bring to your contest will be in your target demographic.
Keep in mind that no media vehicle is 100% targeted. While these sweepstakes and contest sites are not as targeted as some traditional media, the media is FREE and the additional exposure will get your contest and brand seen by more people, increasing brand awareness. Contest marketing will help you reach the right people in a cost effective manner.