I looked in more detail - look at any flight departing today. None of the exit rows for same day departure are green dotted. I did a search randomly and picked SEA-LAS and MIL-CLT.

They're selectable though, meaning non-preferred can book them. I went through a mock booking, without logging in, and it let me pick 11C on a 737-300 HP plane leaving SEA tonight.

So basically, it seems exit rows become bookable by anyone online on day of departure if not already taken by a preferred member.