I booked direct on AA & JetBlue. Both times I booked I got a seat assignment at the time of booking (after reading the replies on this thread, I checked my emails( but when I checked in at the airport, I was told my seat would be assigned at the gate.

I did book my trip to Bangkok on Expedia but it was an ANA ticket.

So, based on this experience, I assumed it had something to do with US rules but it seems like, based on others' experience, one needs to be vigilant about checking your seats to make sure you keep the ones you have been assigned and, unless you are a special frequent flier, you may be at the mercy of the airlines.