The crime: they were aware of malevolent use and didn't report it.
Discussion
If I were a knife, gun, vehicle, or alcohol manufacturer, I know my product is used incorrectly, that is to say for unethical, harmful, or illegal purposes. Would I be responsible for its misuse?
If you knew of specific crimes then yes you would. Knowing of them and not reporting them makes you an accessory.
It would be aiding and abetting, or criminal negligence, not laundering per se. "Tornado Cash does not pose any barrier for people with criminal assets who want to launder them." (from Coindesk article) The root of the issue is if Tornado Cash ought to have a built-in barrier, but this would also make it vulnerable to tyrannical control, like a remote shutoff for vehicles would make policing "too easy," if you will, and vulnerable to misuse and abuse by the authorities.
It can be argued whether it is ethical to build a technology with no shutoff or censor mechanism, but that's basically the same as Bitcoin itself.