If there's a chance that additional evidence may result in an improved probability of conviction, assuming existing evidence would result in a weak case, then electing not to charge Winston now is a good idea.
Peculiar case all around. The fact that the prosecutor was so equivocal in his use of language (including calling the young woman the "victim") means I look at Winston as a likely guilty but not convictable felon-in-fact. I know some may find this position to be objectionable, but it's the way I see him. I don't see the kid as "not guilty" unless and until a jury adjudges him so.