Seiza is a formal Japanese sitting position where the legs are folded underneath of Seiza. (Check out Wikipedia for a more in-depth description of Seiza!) In my opinion, Seiza is the perfect posture for playing shamisen for a variety of reasons.
I should say again, the following ideas are solely my opinions and assumptions. Individual experience might be different based on personal body type.
With your legs folded under you, your hips are positioned in a way that makes maintaining a straight lower back very easy. With a straight and comfortably relaxed lower back, straightening your upper body posture is a simple matter of awareness. (Note, feet don’t have to be folded over each other if that’s too painful)
When crossing your legs (shown in picture below), your lower back is already bent and will require constant, conscious effort to push your upper body straight.
With an erect lower back, extended chest and shoulders straight (not slumped forward), your whole body is efficiently set up for flowing power into your arm.
