i guess, by the term 'inverted' you meant to say the wing be inverted in shape ( rather than the term 'inverted wing' in aeronautical sense).
The horizontal part of the wing does as much (if not more) work on cleaning the flow as the vertical part. If the wing was to be inverted the horizontal part will end up lot higher and the clean flow does not necceserily hit the rear wing as much it would have been had the wing been as it is now.
The air is more disturbed nearer to the surfaces; and thus more disturbed lower down than near the top end of airbox. Any turning-vane-sort-of-element would therefore be a neccessity lower down ( though airbox inlets does produce detrimental vortices , but so does cockpit )
Also,maybe, the stresses put by attaching this wing to a higher point in the airbox(which is hollow) is a bit too much for the engineers' liking.
A possible advantage i can think of the inverted configuration is if the tip(the top most tip) produces any vortex. In such a case the vortex flow would have been directed to the suction side of the wing, whereas as of the current configuration its more likely to end up on the pressure side. But, these airbox wings being purely 'air cleaning'(sorry..couldnt think of a better term.
) devices there might not be enough pressure differences between the inner and outer surfaces to induce any significant vortex.
Between, I cant see this as being a stupid idea .
.innovations more often than not starts from the 'basics'....There might be some advantages that i couldn't think of... Would be great to hear more on this.