Default Calendar Setting won't take

I keep trying to change the default Calendar in the Calendar.app preferences, from Calendar to BusyCal. Every time I relaunch Calendar.app, and look in the Preferences it has changed from BusyCal back to Calendar. I'm getting inconsistent results, where sometimes an attached invite will launch in BusyCal, sometimes in Calendar. When I checked the Get-Info window in Finder on an .ics event, it currently states the default application to launch with is BusyCal.

Is there something wrong with the Default Calendar setting, is it totally defunct, is it working (changing it changes the default calendar .ics files launch with) but the Calendar preferences simply can't show the pre-existing setting?

This is happening on all our office computers in 10.0, as far as I can tell.