Agreed, it isn't a valid argument. But it is a situation that does occur, and people can feel entitled enough to get agitated.

Letting cabin crew reseat people, when there are more comparable seats available, in my opinion, is generally an acceptable thing especially if it is to defuse a potentially volatile situation. Would you really want to create a situation on board that could potentially escalate? And yes, this might include seating someone next to a racist passenger; if a solution can be found that avoids the situation from escalating post-departure, where passengers can't even be deplaned easily, then it does make sense. I know this is an extreme example, but El Al, for example will even reseat women when passengers of a religious nature refuse to sit against them. Is it ok to do this? In my view, from a fairness and equality point of view, no, not at all. But this is not the only thing that matters in these situations, safety does too and it is better to avoid a situation from escalating in the air, than reseating a passenger twice.

Again, the crew could've made the woman feel better about it with more subtle personal touches. That would have likely diffused everything completely. It seems this didn't happen, and could be a point where they need to have more training. I don't think this is an issue of racism, but one where more empathic customer interaction in a tough situation could've resulted in everyone feeling better about what seems to have become an emotionally charged situation.

I am by no means taking side of the staff, but it is easy for all of us to point fingers, and for passengers to make accusations, but do we know what was said to the FA? Maybe whatever this interaction was, left the FA in discomfort enough that a decision was taken to reseat the passenger.