"Might be used for illegal purposes" is a significant understatement. The chief selling point of Tornado Cash is money laundering, which is in and of itself a crime in both the US and Netherlands.
Normally, there'd be an aspect of plausible deniability: torrent index operators can, for example, rightfully claim that they're facilitating legal filesharing, or that they're entirely agnostic to the content being shared (if all they're doing is sharing URLs). What's key in this case is that law enforcement claims that Pertsev was aware of the crimes his service was being used for. Whether or not that's actually true is up to a court to decide.
Normally, there'd be an aspect of plausible deniability: torrent index operators can, for example, rightfully claim that they're facilitating legal filesharing, or that they're entirely agnostic to the content being shared (if all they're doing is sharing URLs). What's key in this case is that law enforcement claims that Pertsev was aware of the crimes his service was being used for. Whether or not that's actually true is up to a court to decide.