You either accept both Bitcoin and Monero as fungible, or you blacklist the former and delist the latter (due to your inability to track it).
You can't expect they will use their common sense because they're using double standards.
If you received coins that they said "dirty coins", your coins will be frozen and you will be asked many information, but in the end you can't access your coins.
But if they i.e. Federal Reserve confiscate the coins from criminals, they can auction the coins and they will earn money from that. Federal Reserve is actually a legal mixer since they launder from "dirty coins" to "clean coins".