I have lived there and go there very often. I think it's cultural. Thai people and especially women are obsessed with weight. You don't see many Thai ladies who are over weight. They generally comment on the weight even among themselves. So I am not surprised that it happened. What bothers me is how it happened. The bringing out of the measuring tape and the public humiliation of the passengers.
It could have been done quietly and out of sight of the public eye. Or they could have upgraded them to first class too.
Then the compensation of just the fare difference from business to economy is really insulting to what they have to face.