It's pretty silly honestly, asking for unpaid work. You don't ask a contractor to do a wall framing test, you hire them and they do it. Blizzard wanted me to do an unpaid art test, but with their reputation, I don't think they are in the position to bargain.
Yeah I agree that art test can be fairly annoying. It's requiring you to put in more time for a job that you may have little chance of getting. It's similar to having to submit an audition tape for an acting role, except I would argue is even more involved.