Then she (and the rest of Federation and their allies') is lucky that Seven never wanted to leave again... In reality, Seven would find herself the unfortunate victim of an exploding shuttle or a botched "whoops, we lost electricity there for a bit" transportation *after* she was allowed to leave, as it was her right. For that matter had Seven really wanted to, she could have assimilated the entire crew and simply flew Voyager towards the Borg. Janeway was damn lucky Seven got on with the program within an episode or two. Like you said, Future Kim, Chakotay and that lady that was with them were charged with treason on their intent but no one threw them in jail and tossed away the key *before* they set out to do anything, which is what Janeway would have to do if she truly wanted to stop Seven from leaving.