There isn't really a way around this.
You literally define a node in the importer set up, if you define a node that is public or get the importer to create one for you then obviously all public and private blogs are going to be imported into it and it will be visible or not depending on the permissions of that node.
So this isn't strictly speaking a bug and something that should be cleared up between completing the import and re-launching the forum.
Blog centric add-ons that have importers may handle these cases differently and may yield a result that is closer to your expectations.