When attempting to insert a link to a Starteam URL (prepended with "starteam://" rather than "http://"), Confluence tries to be smart about the link and strips off the protocol ("starteam:") from the link and treats it like a relative hyperlink to another page in Confluence.

The only work-around I've found is documented here:
https://answers.atlassian.com/questions/31829/confluence-support-non-http-ftp-hyperlinks

Essentially, by creating a shortcut link for Starteam, I can type in "[<server_name>/<rest_of_starteam_link>@starteam] and it returns with a clickable (although not ideal) link that follows the link through.