I'm a flight attendant, and I see it all the time. I always tell the squished person that they can move to another seat if they wanted. I feel so sorry for the people who are squished. The agents are the ones who are supposed to charge obese people for an extra seat, if they can't fit in one seat.
I almost had a man removed because we were a full flight, but the person beside him said they were fine, then found out it was his wife. We just don't know if the person is with that other person or not, and we are taught that we can really only do something if someone complains. It's not in our regulations that they can't take up another seat, but it is in our regulation that the arm rest on the outside have to be down for take off and landing, and the flight attendants should have had them removed because of that reason. But most of the time, we are rushed so badly to get an on time departure that it gets over looked and by the time it does we would have to go back, and most of the time no one wants to.