1c logic does not allow put one item in hierarchy belonging to other owner. Flat representation is very strange to see and very confusing when there are a lot of items. Almost all the time the list is filtered by the owner, so there could not be any problem with hierarchy dependency.
Also 1c support representation of all the items grouped by owner, but it shows all the items flat. In this case I really can not imagine why it is not possible to show hierarchy inside the group. It is like having several list each one filtered by the owner.