Ok, you’re taking about “Basic” seats. Those are not regular Economy seating. Everything other than checked bags is included in Economy seating and the price shown is what you pay.
Right, but I wasn’t talking about regular Economy. The prices you’ll see at Kayak or other consolidators are the lowest fares the airlines offer, and they’ll be the Basic prices, not regular Economy.
Many foreign airlines too, nowadays. I just booked some tickets on British Airways and had to pay to select my seats.
I booked flights through AA, but it was a codeshare with BA. The fare would have included seat assignments on AA, but BA was responsible for the seat assignments and they wouldn’t give me free seats until 24 hours before flight. I cancelled the tickets (within 24 hours, free immediate refund) and rebooked on different flights from AA and got free advanced seat assignments.
Read the fine print.