It doesn't do it consistently (I clicked on the link on the word "entry" to get here, and it stayed in the same tab), so it shouldn't do it at all. The inbox isn't email.

If somebody follows a link from an item and it's marked as read on the database, all they have to do is click "Inbox" again instead of "Back" to see the new version.

If you're using Firefox and you want something to open in a new tab (or window, depending on your settings), you can control-click. Opening new windows without asking is an accessibility problem.