They all confessed, so I'm not sure what your point is.If you want to use the presumption of innocence like that, then you cannot say that Carolyn Bryant lied about Emmett Till. You also can't say that Roy Bryant and J. W. Milan killed him.
These things are crimes. No charges were brought against Carolyn Bryant, while the other two were found not guilty of both murder and kidnapping. They are therefore presumed innocent, and treated as such, in the eyes of the law.
What about Emmett Till? Should he have been presumed to be guilty?