Audiences crave what they're not allowed to see. When you're a kid, there's an undeniable thrill in sneaking into your first R-rated movie, but even as adults we're suckers for the forbidden. That's why we can't get enough red-band trailers—oooh, extra swearing and nudity!—and why we'll buy the "unrated" DVD so that we can check out, supposedly, what was "too hot" for the theater.
While this gimme-more tendency largely applies to raunchy comedies and horror movies, action flicks work under a similar principle. For as much as some people complain about the amount of violence in action movies, studios actually curtail the goriness a little just so that they can make sure their films don't get an R, which would severely limit their potential audience. I'm speaking very generally, of course, but on the whole Hollywood action movies are "violent" without really being, y'know, violent. They're loud and frenetic with lots of explosions and crashes and shooting, but there's very little pain or blood or other things that would make you realize that violence in real life is not this much fun. This weekend's The Hunger Games is a perfect example: The movie's brutal fight-to-the-death games are mostly shown off-screen or shot in a way that we don't see much of anything. It's violent, but sanitized.