Well then they really should show with a lock symbol (read-only). Looks like this may be a (different) bug that you are experiencing. (and since you are only shown one emblem per icon don't see any difference to the list view icons either)
For my part (and this bug) it turned out that the issue is just with the list view not showing all emblems (or overlaying them) and thus making the icons indistinguishable. In icon or compact view the difference is visible.
(I noticed that the lock emblem shown for private/ seems to be OK (contrary to what thought and wrote before) because user1 is in the user2 group but not itself user2.)
>The directories of user2 aren't group writeable.
Well then they really should show with a lock symbol (read-only). Looks like this may be a (different) bug that you are experiencing. (and since you are only shown one emblem per icon don't see any difference to the list view icons either)
For my part (and this bug) it turned out that the issue is just with the list view not showing all emblems (or overlaying them) and thus making the icons indistinguishable. In icon or compact view the difference is visible.
(I noticed that the lock emblem shown for private/ seems to be OK (contrary to what thought and wrote before) because user1 is in the user2 group but not itself user2.)