Why are anime in the US given very harsh ratings?

I've been realizing that anime nowadays have been given very harsh age ratings. Anime such as FMA brotherhood, Code ge, Hetalia and many more have been rated tv-ma. I know its cause of the violence and stuff, but I've seen gorier and more graphic tv shows (such as Heroes and Supernatural) that get only tv-14. And some episodes don't even qualify for tv-ma (the last 2 episodes of code ge R1 had tv-14 content but got a ma rating). So if live action shows like Heroes, Supernatural, Prison break and many more can get a tv-14 rating while being able to show extremely graphic violence, then why can't anime? I mean its only cartoon violence so why rate them tv-ma?