Well, it does put them in the calendar, it just doesn't allow you to RSVP.
I've changed my opinion of what the RFCs say should happen, based on RFC 2446,
sec-3.2.2.6,
sec-3.2.3, and
sec-4.2.8. These all indicate that it's "legal" for someone not listed in the original attendees of the VEVENT to Reply to it.
On this basis I just submitted this as a bug to Apple, #7026103.