I decided to do more experimentation as to which seats appear to be affected by this change and now come with a $15 price tag and which seats do not. As I originally noted, the Coach Choice selections are fairly bizarre.

Making dummy bookings while logged in as a Platinum elite on wide-open flights approximately 90 days out, this is what I found . . .
Just bizarre.

On most narrowbodies, one random row of foreward aisle seating now costs money, but not the bulkheads, which surely are more appealing.

And the entire right side of the exit row on the 319 (including the middle seat) comes with a charge, but the entire left side is free.

Meanwhile, on the 320, the middle and aisle seats of both sides of the reclining exit row are tagged for this surcharge, but the window seats are free.

On the 753, its two random rows of forward aisle seating that come with a cost. And the entire over wing exit row comes with a price. But row 40 is still free.