Unless I'm mistaken, I really don't believe it's an issue with the spec. There's nothing in iCal (RFC 5545) that says you have to take iCal objects attached to emails and automatically include them in the calendar. In fact, iCal servers are simply configured to accept the xml requests and distribute a list of VEVENT objects (among others) for display. They aren't inherently connected to a working inbox.
My guess is they have loose rules around auto-adding iCal attachments detected in IMAP/Mime messages. It's notoriously difficult to get email and calendars working. There's a reason ActiveSync is so damn expensive.
Yeah - At FastMail we default to only adding if it comes from someone in your addressbook. You can reduce it to just an addressbook group, or open to the world if you feel like living dangerously.
My guess is they have loose rules around auto-adding iCal attachments detected in IMAP/Mime messages. It's notoriously difficult to get email and calendars working. There's a reason ActiveSync is so damn expensive.