It seems like there are three strategies airlines take when it comes to middle seats in Y+:
1) Elite satisfaction. Make elites happy by optimizing for empty middle seats near them. See: BA's "theoretical seating"
2) Advertisement by experience. Give away nicer seats to low fare/non-status pax, in hopes they like it and pay next time
3) No strategy. Don't optimize for anything and fill seats front/back (or randomly or whatever)
My guess is AS is probably just #3 and hasn't bothered to spend time optimizing for #1-2.