If you look at this page, most of the tires shown have the front tire with grooves pointing up and on the rear, grooves pointing down.
There are a couple that seem to show no direction and the Shinko 230 which does it backwards. The Pirelli also has the front down facing but the rear sort of goes both ways.
Bottom line is; it probably is no more than a fad. Today's "look". Like black wheels and "Transformer" styled bikes.