Actually it does, because there are so few good series that are not anime, so it is simpler to just add them to the category. Plus the fact that Samurai Jack is done in the style of anime, similar to Panties with Stockings and Guarterbelt. Avatar and The Legend of Korra technically aren't anime, but they are still included because of their style.
Samurai jack is NOTHING like Panty and Stocking with garter belt. Plus, saying that there few good series that are not anime is a statement of pure ignorance. I'm more than willing to agree that Samurai jack is heavily layered with Japanese influence but it's still not an anime.