That's right, upwards. But for example Christmas trees are almost always drawn with branches going downwards, and I suppose that's created a strong mental image in the back of people's heads that they tend to draw or model evergreens that way. That sort of a structure is taken as given to the extent that artists often don't even bother to check what they really look like.
Well, usually tree branches tend to grow upwards in hopes of getting more sunlight (or in some cases they way it suits them better for getting more sun), but Christmas trees usually are loaded with balls, stars and random decoration, and that pulls the branches down.
So in a way it is understandable (at least why they draw the festive trees that way), but yeah, sad how they don't bother to check references with stuff they think they know how it looks.
Then again, not many people know how trees look like (perfectly at least) or actually bother to pay attention to in-game graphics (outside of the "whoa looks realistic" or the like).