Regarding Make Query Table event layers:
If the first table in the list of tables/joins is 1:1 (i.e. only one row per record), then the columns from that table should be editable.
For example, that's how it works with Oracle views (outside of ArcGIS): views are editable via SQL if the data is 1:1.
Reason: We have use cases where we want to query data for the purpose of filtering certain records as well as utilizing supporting information from other tables. And we want to edit the data while doing it.
Yes, that can be achieved with in-map joins for simple cases -- such as only joining on a single field. But for more complex cases like joining on multiple fields, a Make Query Table is the only option. So it would help if we could edit in the resulting Make Query Table event layer.
Note: Other event layer types such as route event layers (linear referencing) are editable. Why not allow editing in Make Query Table event layers too?