This is a good point that you raise and we are aware of some limitations with the functionality. The requirement to update bus contents through the design using a manual process is to ensure that there are no schematics/sheets locked by other users that would prevent a successful update, this is due to the built-in concurrency available in the tools. However, if you are the only user in the schematic at the time you can update the bus contents for the entire design using on of the 'scope' options in the 'Edit - Update Bus Signals' command, the scope options are - Project, Design, Schematic and sheet. Project will attempt to update across multiple designs, if part of the design is locked by another user it will skip those sheets. Design is similar but for a single PCB in the project, schematic works at the current level and will work on all sheets and finally sheet will only update the current sheet, other sheets at the same schematic level will need to be modified separately.
On your last point, I agree that we could improve this particular feature.
OK - so the "update bus signals" behavior is not as bad as I understood it - menu workaround should be good enough. It may be worthwhile to include scope options in popup menu. It is unfortunately that support people did not know this feature - it would have saved me some agony.