Quoted:
This has come up on the SabMag mailing list a few times.
On common answer is that it reduces or corrects hydroplaning if you lock the front wheel.
Another common answer is the observation that the same model tire made for the European market has the "arrows" in the opposite direction, and that the tread pattern may be chosen more by local aesthetics than any overbearing technical reason.
We're going to need some data to support that one. Michelin, Dunlop, Avon and Continental tires are the same in the US and in other markets. I've never heard of a "reversed arrow" on a tire.
The sipes on a motorcycle radial front tire are designed to move the water into a narrow channel, thus eliminating spray. Look at
this page. These are the tires I have on my BMW. If you notice the image at the top of the page, you will see that the front tire produces a very small and shallow rooster-tail, whereas the rear tire sprays the water higher and farther.