wp_title in author archives

“Why doesn’t wp_title() display the author’s name on an author archive as it does for a category or date archive?“

It’s a good question. But the answer is not to use the_author(), which is intended for use in The Loop. Try this instead:

<?php
if (is_author()) {
global $author;
$name = get_userdata($author);
echo '&raquo; Posts by ' . $name->user_nickname;
}
?>

For a full name, change the echo line to:

echo '» Posts by ' . $name->user_firstname . ' ' . $name->user_lastname;