I'm assuming that "FA standing in exit row" is part of WN's boarding procedures as filed with and approved by the FAA. It is NOT an FAA requirement -- as you may have noticed other airlines do not have an FA at the exit row during boarding.
Plus, the whole exit row qualification is a "newish" rule -- from the 80's or 90's. Ironically, they didn't care back in the 70's when airline accidents were much more common.