I would hypothesize that they are noticing Batman on an unconscious level, but it was not salient enough to enter their conscious thought processes. And therefore if you replicated the experiment with an interruption to the daily routine that does not have superhero morality connotations (e.g. the gorilla suit) the result might not be as pronounced, and not as many people would give up their seat to the pregnant lady.

But who knows? Pretty cool study!