Make constraint subtarget picking work like target picking

I find it strange that it is possible to pick target object for a constraint with mouse that turns into eyedropper. However, when one has picked the target (object), subtarget field appears, but for that it is not possible to eyedropper pick an object, one must browse subtarget from a list or write its name.

I think this is very inconsistent UX/UI design, both target and subtarget should be pickable with the same procedure.

Below you can see the fields. Green one is pickable with mouse, bone (subtarget) must be selected from a list.