On Evolution 2.12.3, reading calendar from Zimbra 5.0 server using webcal:// (calendar type "On The Web"), all calendars open in read-only mode, rendering it impossible to make any changes. Using caldav:// (calendar type CalDAV) everything works fine.
They're two different protocols. "On The Web" calendars use the WebDAV protocol and are read-only. CalDAV is a newer protocol that builds on WebDAV that allows you to edit web calendars, if the calendar server supports it. The evolution-caldav package provides Evolution's CalDAV implementation.