It's by design. See here:
EDIT: Original link was broken and I can't find an updated version of that page. Here's another that says the same thing:
http://blogs.technet.com/b/ewan/archive/2012/04/20/tip-o-the-week-110-tracking-outlook-responses.aspx
If you're invited to someone else's meeting, you'll see options on how to respond, and you'll be able to look at the scheduling view to
see who else is on the list, but you won't be able to see how they're
responded to the invite (well not entirely). You may be able to see
the details in the scheduling view (depending on whether the invited
attendees have given you the permission to see their calendars).
If you didn't organise the meeting, you may be able to open the calendar of the organiser and still be able to see who responded and how.
