What the article doesn’t say what they are charging him:
littering for leaving the shell on the ground.
Early reports were that the police recovered Zimmerman's semiautomatic pistol with "a full magazine and an empty cartridge case in the chamber". About the only way that could have happened is if Martin grabbed the gun in his left hand and tried to wrestle it away from Zimmerman. If Zimmerman had his finger in the trigger guard it is possible that Martin caused the gun to fire by jerking it away from Zimmerman and towards his chest. Martin's hand would have been holding the gun by its slide, thus preventing it from cycling and ejecting the empty cartridge case and reloading a live round from the magazine.
The lack of an empty shell case on the ground says a lot about what happened during the struggle, two people had a hand on the weapon when it fired. Martin's left hand and forearm should be checked for nitrate residue, and the slide should be checked for fingerprints other than Zimmerman's.
Regards,
GtG