Although not the historical reason, a far simpler one is: you can change your mind and ideas but can't change your race.
In addition, it is that behavior and not that belief that gets the racist removed from the premises. You can choose to be quiet about your ideas where they are not welcome, but you can't choose to not be your race where it is not welcome.
So we can ban people for/against atheism because it's an idea one could theoretically change their minds on?
Atheism is literally the opposite of religion.
Did you know that the incident in question is about this Sargon fellow attacking white nationalists? So to further kill the analogy, you kicked a fellow out of your pub for saying mean things things against racism while at another pub.
But then a world in which race was a choice would be so different from our own that one would wonder whether racial discrimination would even exist, or what "black" would even mean.
I am not saying I agree or disagree with it, but perhaps things are not as hypothetical as you believe.