While the entire seat map is open for assignment at the airport, there is still a responsibility on the part of the agents to seat only full-fare or Premiers in E+ and, if possible, leave the center seat in these rows open.
I have been gracious enough to seat very tall and/or larger people in exit rows when I know that the flight is wide open and all Premiers are seated comfortably regardless of their status or fare paid.
However, E+ is a valuable product for UA, and I always make a point of the exception being made and wish them an enjoyable flight.