Why can't an Editor edit defects, user stories and other artifacts?
An Editor has the option to work in 'viewing' mode. Under that mode artifacts can not be edited. This mode is made for convenience of the Editor user which mainly is used as a cautionary measure not to make undesirable modifications, especially if many windows are open.
The 'Editing' or 'Viewing' mode is decided by the binoculars icon that's left to the 'Done' button. See screen shot:
Switch to Viewing Mode:
Getting to Viewing Mode: