Steps to reproduce:
Note that double-clicking foo.txt in the sidebar does make a tab, as does cmd-p (though only after the cmd-p dialog goes away does the tab appear).
This is by design
May I politely ask, WTF?
What possible reason could there be for having two variants of open?
Single click is transient, so it's more like a file peek instead of open. If you double click, cmd-p, edit the file, etc it actually opens. You can use it to glance through a bunch of files quickly.