As far as I know, there is only one eyewitness to the actual altercation, and he insists that Martin was on top of Zimmerman. He also insists that it was Zimmerman who cried out for help.
I don't know how anyone who wasn't watching the altercation could know who was crying out. None of them knew Martin or Zimmerman. Just because Martin was 17 doesn't mean he had a higher pitched voice than Zimmerman. That is a ridiculous assumption to make.
It's easy to say that the police screwed up if you aren't familiar with police work. The police are probably well aware that witnesses often give contradictory accounts. They probably gave greater weight to witnesses who saw the event in question, and less weight to witnesses who were speculating about what they heard. None of this means that SPD is racist or biased.