Well, there are 2 sides to every story. Taking what you say at face value, however, it's a rather unfortunate situation.

Sure, sounds like a stupid rule. Still, you did argue with a flight atttendent and refused to follow an instruction to fasten your seat belt. Again, unfortunate situation.

For what it's worth, I have flown ATA several times and have never had a problem. FA's have always been pleasant and have always allowed me to switch seats. Good thing you followed through though!