Yup. Different rules in the USA.

I was at an exit row DFW-ORD when an obese passenger came and wedged themselves in the middle seat. Literally ‘wedged’ so that the fixed armrests with tray table inside were pushed out in a ‘V’ shape. I couldn’t sit in my seat.

I went up to the galley and they refused to see the problem. They said the rules require them to ask if the passenger is willing and able to assist, and if they say ‘yes’ there can be no further assessment or questioning of the passenger (discrimination or law suits or something).

As we were going no where because I couldn’t actually sit in my seat, a passenger from first class eventually offered to trade. But not without the passenger next to me rebuking me that I had made them feel uncomfortable and hurt their feelings.

Even when the passenger got to first class their shoulder and top half protruded into the aisle.