Did just a little bit more investigating and discovered that the AS nonstop flight from SFO to PVR has six seats in F that are unassigned at the moment. AS and VX doesn't give the option to purchase seat on this flight as well (3/31/2018).

Guess it must be some sort of group purchase by TA or Corp. I'm thinking it would be odd for three couples or six people to no select a seat. VX is still offering to sell four seats at the $999 fare on their outbound flight though.