This behaviour is intentional. A tar archive does not contain a central directory structure, it is just a chain of files. As a side-effect it is possible to have multiple files with the same name in one archive, e.g. when append mode was used. That's why the archive must be scanned from the beginning to the end as soon as you reference an archive member by its name.
The best way to deal with this issue in my opinion is to improve the documentation for the stream interface.