Wow! Another dimension of the story and I never thought or reasoned it from this angle. My idea of the reason exchanges seize or reject their so termed unclean bitcoin is because of the regulations imposed by the government in whose country they operate.
I have long thought that it is the war of the government vs the people and exchange. But right now you are making me believe the exchanges are even the master minders of all these.
But I still have my doubts. Why should an exchange reject a coin or frustrate it's flow knowing so well they will make money from the movement or trading of the coin?
This is not a global conspiracy. There is also the influence of governments who want to have the same control over bitcoin that they have over fiat currencies. And ersatz bitcoins in the accounts of the exchanges allow them to control the bitcoins of those who store them on the exchanges. Therefore, both exchanges and governments are interested in keeping bitcoins on exchanges. The common interest contributes to the fact that they work in the same direction, while without any secret conspiracy, all their motivation is in plain sight.
All ersatz-bitcoins on the accounts of exchanges, banks or any other are inherently fiat. The real bitcoin is only on the bitcoin blockchain.