I can't change the behaviour without editing the code behind the board, which would potentially make future version updates flaky as hell, so I'd rather not.
Meanwhile if you're using a vaguely modern browser ctrl-click will do the same as right-click->open in new tab.
If this is really bugging a lot of people (add a me too here if so) then I'll go ahead and change it; it's just a fair bit of hassle.
Other sites based on older versions of PHPBB will potentially do the target="_blank" thing, but that's generally not considered good form these days by purists. :-/