His sentence was commuted.
http://www.nytimes.com/2010/12/24/nyregion/24commute.html
Another issue here is that people outside the U.S can never seem to understand how laws and sentences vary from state to state. New York and Florida are very different when it comes to firearms and when to use them whereas New York is most likely the last place in the U.S where you want to use a gun but his sentence was still commuted.
There were key differences in these cases too so they definitely were not the exact same circumstances. Same for OJ Simpson, it is easy for us to sit here and say something like "the jury let him go", but that was not really the case. It is a tough decision for a jury to convict a person to decades in prison where we dont have to make that decision. Wherever there is reasonable doubt they are going to side with the defense over a conviction and that is the way it is supposed to work. Even serial killers who admit to numerous murders may only actually get convicted for one or two.
During the Casey Anthony trial it seemed like everyone wanted a conviction, but really there was no proof she committed the murder, and you cannot just say "oh well she did it...she had to" because it is the jury that is seeing this a different way.
I watched the Zimmerman trial (most likely you did not) and I cannot fault the process or the presentation.
Actually there is a huge difference, OJ rightly or wrongly disputed the facts of the case, so did Anthony. This racist filth DIDN'T dispute the facts, he proudly told EVERYONE how he stalked and murdered this kid. THATS the difference, if he had said "It wasn't me" then there might be some doubt but HE SAID HE STALKED HIM, HE SAID HE SHOT HIM. THERE WAS NO DOUBT ABOUT THIS