How do I select/move multiple albums or events in the Photos sidebar?
You can utilise album view as prior suggested and then drag albums into the sidebar to achieve the desired result, but you are absolutely correct in noting the failure of this desired behaviour to operate, purely within the sidebar itself.
yup the OSX HCI / UX toolbox in terms of standardisation of elements with predictable results is something that apple has debased since leaving OS9, selectable text OS wide - has similar issues.