Problem Attaching Files to CalDev Calendars

In LION's iCal, I am unable to attach a file to an event when the calendar being used is managed/maintained/stored on Apple's CalDev Server. There is no "Attach File" field available for these calendar events. The only way I've found that this works is if the calendar is created to be stored on my local computer. Is this a defect (bug) or did they design it this way?

If is by design, why? I can't see any reason why files are attachable to events on local calendars but not those on CalDev calendars...