Ok so four charges:

1) Conspiring to money launder

2) Money laundering (so I guess the conspiracy succeeded?)

3 & 4) Failing to register a money transfer service

So the important steps are

a) demonstrating the defendant (Sterlingov) is the person running (and I guess profiting?) from the mixer. That's not a crime, just that it's a thing that they have to do, and I guess we're able to successfully show (sounds like some dumb "opsec" mistakes).

b) show that the defendant intended money launderers to use it, and that they did (charges 1&2)

c) show that the defendant did not register the service (charges 3&4)

So I'm going to leave (b) to the end, it's more complicated and IANAL, so let's look at (a) & (c).

Charges 3&4, and C is very simple - it does not matter that it's online, it does not matter that it's crypto, and it does not matter whether or not you care about privacy or laundering. The law requires all money transfer services to be registered, and this service was demonstrably not registered, so it's a very clear cut that whoever operated the mixer was breaking the law. That the prosecutors had to prove Sterlingov was running the service further demonstrates that it was not registered, and given they apparently were able to prove he was running the service I hope we can all see that he is guilty of this crime, even if you may not like the law requiring you to register your service (if you think a law is illegal you would use the courts to prove it is illegal and invalidate the law).

To me it seems plainly obvious, that given they apparently had sufficient evidence to show that Sterlingov was running the service, that he's guilty of not registering it, even if he had actively prevented any laundering using it.

So that leaves the money laundering charges. Once you're transferring money US law has a variety anti laundering regulations, and the fact that some, or even most of your users are legitimate isn't a requirement. Afaict they just have to show that you could be reasonably aware you were processing illegal transfers and were not actively trying prevent those transfers. This applies to banks as well as mixers. A bunch of forum posts by the operator specifically talk about how other "legitimate, visible businesses" would be forced to reveal info about your funds to authorities (directly comparing its secrecy to SilkRoad), posted links about needing a mixer to evade taxes, and how you needed to use a tor based mixer (e.g. their's) to avoid law enforcement taking down the mixer. It doesn't take a giant leap to see even from what's in the government's statement of facts here that this was being advertised as a service to hide transactions that would otherwise be required to be reported. The government does not need to point to them explicitly saying "hey, use me for your laundering needs".

Note that it does not matter if you think you should not have to report those transactions, the law says that you do, and that a service that facilitates you hiding those transactions is breaking the law. It does not matter if you think some transactions should not be illegal, the law says they are illegal, and that a service that facilitates those transactions if breaking the law. This case is not "bitcoin fog made illegal sales" or "hid its income from the IRS" (though I'd be curious if they did that), it's that they facilitated others doing so in a manner that was illegal.

If you want to run a mixer, it's very clear you will be running a financial transfer entity and need to register it, or you are committing a crime. Again it does not matter if you think it should not be a crime, the law says that it is. If you don't like it, you need a court to rule it's invalid, or you need to get people who agree with you elected.

If you want to run a mixer, it's similarly clear you are not subject to completely new rules (or lack thereof) vs other financial transfer services so you have to try to prevent illegal uses of your service, not just not promoting it for that purpose, but actively preventing such transactions as much as reasonably possible, including identity verification, just as required by any other financial service.