when your car is at speed there is somewhat of a "pocket of air within your wheels that is stationary relative to the air rushing past it. Air in motion has lower pressure than air not in motion, so the higher pressure "pocket" of air within your wheelspace moves outward towards the area of lower pressure in order to balance out. The wheels forcing air in would work against this to some extent. That would have to be my guess why they are designed to draw air out.
I cant really see in those pics whether the shape of the spokes directs air in either direction, but it would be hard to imagine Porsche making such an oversight.