Do you believe the 12-18 year-old set gets that message? I think it more likely they simply incorporate the violence and the acceptance of it because it fits well with all the other reinforcement they get in that direction from pretty much everything being sold to them as "entertainment" these days.
No doubt you'll disagree.
I believe they do, yes There's a lot of YA books written about dystopic futures where it's up to the teenagers to say "hey, this sh*t isn't right" and fix it. Which, if you think about it, is a pretty appealing message for people that age.
I think you underestimate what savvy media consumers kids are these days. Growing up in a media-saturated world, they've learned to tell the difference between (for example) the glossy, sanitized violence of your typical action flick and something like this or Saving Private Ryan . There's no "yeah, take that, bad guys!" music, no catch phrases, no smirking over the fallen antagonists. She doesn't suddenly develop amazing kung fu powers or abruptly go bloodlust-berserk. Katniss does what she does because she's given no choice. She uses her skills to survive in a world that's turned against her. She makes the best of the choices given to her.
I think your cynicism here is preventing you from looking deeper. The Hunger Games is no more about cheering on violence against kids (or by kids) than Nineteen Eighty-Four is about the rat helmet.