It's not just basic economy, though that is part of it. Some other examples: People who book through a TA often don't book BE fares, but still don't get a seat assignment until they check in. Groups sometimes don't get seats until they check in. People who booked the flight as an award ticket through another airline (e.g. using AF/KL miles) sometimes don't get seat assignments. If all the non-preferred seats are taken, then even the everyday Joe who buys a ticket won't get a seat assignment until the gate, no matter how many preferred seats are taken.