Sounds like a bug or oversight in the calendar widget. Can you file a bug report for this in our support system?
I had a quick look at the latest version of the Calendar, which seesm to have been completely rewritten from earlier versions, There are a number of changes that meant that it was easier for me to revert to an earlier version of the Calendar widget. From memory, the ones that stopped me using the new version were:
a) The New Event Reference now needs to be a reference set, where in earlier versions it was just a reference association
b) The Title attribute now needs to be an attribute from the main entity, whereas before it could be an attribute from a related entity. So for example, my appointment is linked to a Company record. I used to be able to use the Company name as the Title, but this is no longer possible.
It looks like there may also be other changes that were not immediately obvious.