Speculative update - I'm sitting at the gate and looking at the flight status. I think a bunch of previous posters is correct, that AS is probably experimenting with something new.
First, the cynical software engineer in me thinks that the email subject of "exclusive" basically is just a sugar coated way to say "we're trying out something new with a small subset of users". Second, the paltry $35 offer (keeps any costs of this "experiment" to the minimum). Third - if you check the flight status for AS1400, they've assigned 10 seats, with 2 people waiting, and if you check with the seat map, there are still 7 seats available on the plane so it's not like this is an oversold flight
Or maybe enough people took Alaska up on a $35 offer LOL
Again, this is just speculation