The Legal Woes of a Superhero

The boys are watching a Batman cartoon right now. My oldest son posed an interesting question:

Batman and Spiderman always beat up the bad guys and leave them tied to a lamppost. How do the police even know they did anything?

If you are attacked by a superhero who then leaves you tied up for the police, I suppose in any legitimate legal system then all you would need to do is deny everything. If no one saw you robbing the bank, breaking into the vault, mugging an old woman, etc. then it's simply your word against Batman, who isn't going to show up in court.

I believe this is a major flaw in the whole Batman franchise. Unless you're stopping criminals who already are wanted by the police, or have been witnessed (and clearly identified) committing a crime, then you really haven't done much except beat some guy up.