Maybe they're trying to use the threat of a life sentence to intimidate and bully him into pleading out.
A politically correct lynch mob. They are charging Zimmerman to appease the mob, even though they know their case is extremely weak. They are raising expectations which is just going to add pressure to the situation. When those expectations are not met at trial, their will be an explosion. Remember, the Rodney King riots didn’t happen after he was justifiably beaten by the police, they happened after the police were justifiably acquitted at trial. That was, of course, before the federal “justice” department decided to subject the police to double jeopardy by retrying them in federal court.