Not at all - the OP had seat assignments to begin with, but lost them when there were equipment changes, also common with Delta. (Read this one: Delta Dumps My Seats, about a family of four with a lifetime SkyMiles Platinum and how the family was split up on four flights and nobody from DL could / would help, so they had to invoke "self-help".)

Most airlines also do not assign seats once a specified number of seats have been sold, and certainly in an overbooking situation, holding back seat assignments for assignment at the airport - as well as having certain seats they do not assign until the day of flight (some handicap or family seats, for example). And a number of airlines are just not sophisticated enough in software or service to be able to do the right thing anyway.