While it's true that foam does not impede or change the effective volume of the air box, it does change its resonance characteristics. You can prove this to yourself by first blowing across the mouth of a bottle to hear it's tone, and then repeating this after partially filling the bottle with filter foam.
Filters located in the air box take-up space which changes the resonance of the intake system at various engine speeds. Air box resonance helps to fill the cylinders with air, and it’s important to note that factory engine development work to smooth-out the power and torque curves is done using the stock filters. In particular, air box filters degrade throttle response, the ability to smoothly accept changes in throttle, by reducing the volume of “free” air located in the space between the filter element and the throttle plate. The volume of “free” air should be at least 1.5 liters for the best throttle response for liter displacement bikes.
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