The real question is why athletes have to represent a country at the World Championships or Olympics in an individual sport like Track and Field.
You would think the "World Championships" would be a meet that tried to have all the best runners in the world present. I guess it is more important to have more countries present than it is to have the best field possible. (Hey I'm not complaining, I would be far less invested in championship races with 12 Kenyans and no Americans).
This is a smart move by Parsons as he is now much more likely to run in the Olympics, which even the average person is going to be impressed with. I am sure he has some German roots and his family over there is excited, but if the move didn't make sense financially and from a career standpoint, I doubt he would have done it. Same with Pappas, same with the WCAP guys.
I do think the difference in responses to Parsons and Pappas is really just about whether you liked them before the decision or not. That doesn't mean it is sexism.