What is interesting here, though, is that Tornado Cash doesn't run anything: they may as well not exist as an entity anymore. It also isn't a blockchain and doesn't have nodes: third parties thereby also don't run Tornado Cash.
Tornado Cash, instead, is one of many random contracts executing on Ethereum (as well as third-party Ethereum-like constructs; some/many of those are pretty damned centralized, with servers that process and store transactions, so I am gong to concentrate on Ethereum itself as it is the most interesting).
So, with the smart contract of Tornado Cash -- as Tornado Cash isn't a group of people or a company: it is just code -- having been sanctioned, if you are a miner and mine a block on Ethereum that includes a transaction that touches Tornado Cash, is that now illegal? I feel like this is how the concept of sanctions would normally play out in meat space with real people, to prevent the sanctioned services from being utilized.
If so, and I think this is where it "gets good": let's say 99% of miners care about the sanctions, and 1% don't... when you mine a block, you choose a parent for that block; arguably now, if you actively chose to accept a parent block that includes a Tornado Cash transaction, that should also be illegal (due to being sanctioned).