What controls the View button displaying in the Event Material Detail Page in CRM Online?
If an extended page layout is used when viewing an event object, then the display of the View button should be based on the layout configuration of visible buttons. In addition, the View button is made visible.
If an extended page layout is not used when viewing an event object and the fallback SFDC page layout is used instead, then the View button does not display.
CRM Help Documentation: Managing Event Material