1. Because Batman is Ben Affleck.
2. Because if Superman wins, it won't be a very good introduction for Batman.
3. Because probably kryptonite will hurt Superman a lot, and even if it hurts him less than in the old movies, then he will be strong enough to keep on in a battle where most likely Batman will win at least once
4. Actually I really think that they will get even, independently whether they will fight once or twice by score 1 : 1