Is that what they charged him with?
No the jury had three options from 1st degree down to manslaughter - so they had the option of involuntary manslaughter. Even if it was not on purpose (the defense) the reckless handling of a fire arm in a crowded public place should have gotten him manslaughter.
As other have pointed out, I suspect the fix was in and the prosecution may not have been as strong as it could have been.
I do not know one way or another but it is strange such an open and shut case (video and confession) would lead to a not guilty verdict