Or maybe it's rated Teen because nothing has to be legally rated in the USA so you guys get a bunch of self-rated bullshit that's all wrong. If it's Viz who gave it that rating, then they probably didn't even watch the DVDs and just assumed it'd be the exact same thing as their manga.
On the other hand, it looks like they might not have a rating between All Ages and Teen (how nice of them not to have a glossary). Since One Piece has violence, it doesn't really fit all ages I guess.