While I agree with your point, your argument is not really valid. In FPS games, 'up' is usually mapped to walking forward. That works differently in 2D game :-P
I'm just saying that it's not a farfetched or backwards way of implementing it. It's just a simple choice. In the gaming community, it's an intuitive binding for many.
What I meant to say is, in FPS games it's usually Space because 'up' or 'W' is not possible, that's taken for walking forward already. That being said, I agree with you that Space for jumping is not necessarily a bad choice in 2d games.
