Why Is the Event Attendee Business Rule not running on the Attendee Hub page and only on the Event Action specified in the Event Attendee Business Rule?
The Event Attendee Business Rule will run before adding attendees from the Attendee Hub page. When the attendees are reaching the exact Limit_Quantity_vod__c defined in the Event Business Rule, no warning message will appear when adding the attendees. This is due to the Attendee Rule Process not being able to verify the Attendee_Status_vod__c and Attendee_Account_Type_vod__c values for the current Event, if specified. If a Run_Rule_Event_Action_vod__c is specified on the Event Attendee Business Rule, the warning will appear when performing that Event Action.
A warning will appear on the Attendee Hub page for the next Event the attendees are added.
CRM Help Documentation: Event Attendance Rules