to develop FBD for this system, the wall reaction will be included from 1 side only! why we don't include the reaction from both walls at both sides?
At the point in the rotation as it appears in the illustration, the horizontal force of the piston rods on the piston tend to want to push the piston to the right: i.e. towards the right side wall and away from the left side wall. Hence there will be a non-zero reaction force from the right-side wall on the piston, but no reaction force from the left-side wall on the piston. Later in the cycle, the opposite will hold.