beren
Graduate Poster
Incorrect, the US legal system has no definition of "not proven guilty". It has two settings "Guilty" and "Innocent". When not proven guilty, Innocence is assumed.
Here is what the jury instruction in the Zimmerman trial said. Emphasis mine.
If you find that no offense has been proven beyond a reasonable doubt, then, of course, your verdict must be not guilty.
Source: http://www.flcourts18.org/PDF/Press_Releases/Zimmerman_Final_Jury_Instructions.pdf
Zimmerman is "not guilty" because his guilt was not proven. This does not mean that he did not commit murder. It merely means, that it has not been proven.
Yes, legally, this means that he cannot be considered to have done it, but this does not make him "innocent." It means he has not been proven to have done it.