do people not realise that ratings aren't just for sex, language and other content affect it too?

A-ha! There's your problem. In the USA, sex is way, way more likely to get an adult rating than violence is. So that's the default assumption.

It's why I refuse to use the MPAA system. I just go for "All ages", "Mature" and "Adult" (and I never use the first of those anyway).