I don't know why most if not all forum software set default logo link to forum's index page instead of site's main page, but from I can see in latest phpBB demo under board settings there is:
Thanks.Main website URL:
If specified, a link to this URL will be prepended to your board’s breadcrumbs and the board logo will link to this URL instead of the forum index. An absolute URL is required, e.g. http://www.phpbb.com.