Can't select same owner as Story when creating Task

While viewing an Iteration Plan in the Web Interface, I clicked on the '+' sign next to a Story to create a new Work Item. My goal was to create a Task for this Story for development work.

The Task form appears in the Plan view as expected. When I select a Task Owner I try to select the same person who owns the Story but the name is greyed out. All other names are available.

Is this expected? For us, the Owner of the Task is usually the same as the Owner of the Story.