I had the same question and jd52cranbrook who has helped me a lot understand this system answered this-
Its the line into the intake manifold that creates the suction for the circuit. I understand your thinking the carb would suck the air in the opposite direction, but the air coming in from the air cleaner has so much more volume, and more free flowing that it does not. However the tube or hose into the intake manifold is much more centered, kind of like sucking through a straw rather than just breathing in from a open mouth.
Hope it makes sense, sometimes I confuse myself, lol.