> A federal "money laundering" charge requires an illicit origin, and so since successful money laundering will never have an illicit origin, the charge can only be tacked on to another indictment.
Fair.
But think about it: a _singular_ person in your mixing pool can make the entire pool illegal. One, singular, person using that money for contraband (illegal porn, illegal drugs, illegal tax evasion) will turn the entire pool illegal.
Do you trust everyone else in the pool to be using it for legitimate gains? If you went into court, would you be able to say with a straight face that "Everyone in my mixing pool was in fact, doing legal activities?"
So in practice, I'd argue that most mixers are illegal (because surely, there's at least one person using the mixer for illegitimate purposes). Furthermore: the BTC transaction into the pool (and out of the pool) will forever be written into the blockchain. If you use your same wallet for both sides (or if the prosecutors can prove that one of your wallets was on the input-side, and another one of your wallets was on the output-side of the mixing process), you're now tied to all of the illegal activities that mixing pool is associated with.
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That's the thing. Mixing pools have never been tested in court. But imagine what a prosecutor would say to a jury, and imagine what the jury would rule.