I love how people step in to defend movies whenever someone (rightfully, in cases like this) complains that a movie is PG-13.
I think no one ever said that they'll suddenly start hugging instead of fighting and shooting, but considering that The Expendables franchise's big logline is that it's the return of old school action (neither the first or second one succeeded in doing that, IMHO), the PG-13 rating is out of place.
People bleed (red blood, not some invisible powder) and sometimes curse in old school action movies, even if it's just for a few times. I constantly see posts like The Matrix could be PG-13 if they left out some words and cut out a few seconds - well, they're not. And those few seconds of violence give them an edge a PG-13 movie would not. Same with movies like Die Hard, True Lies, Lethal Weapon and Speed. Are they ultraviolent and extremely vulgar? No, they're not. But those little slices of violence and hard-boiled moments give them an edge PG-13 simply would not.