Presumably they mostly stole ETH because tornado cash is the best mixer around to launder stolen funds
I'd assume any attacker would at least transfer everything to a BTC/whatever address generated offline, then figure out later how to launder it.
Since zero-knowledge cryptography is used to ensure the generated note cannot be linked to the depositing transaction, it can be used to send money to yourself or another person without revealing the identity of the sender. There are criminal and non-criminal reasons to do this.
Because it is a smart contract system, you do not have to trust a person or organization with the money. You do have to trust the smart contracts defining the system are correct. The smart contracts are publicly available to read and have been reviewed by many people, including software audit organizations.
To expand on that, say someone withdraws ETH from Tornado cash and purchases an NFT with it. The seller of the NFT then swaps their ETH for USDC on a decentralized exchange (the ETH then goes into a pool). Later, a liquidity provider to the ETH/USDC pool withdraws liquidity from that pool, and sends their ETH to an exchange, let's say Binance. If Binance blocked such deposits (and especially if they did so without refunding the user on-chain), no one would use Binance, and they'd also be the target of a lot of lawsuits.
The compliance topic is tricky and deceptive. Only the user with a "Note" is able to link deposit and withdrawal. With this note the user can generate a proof of origin. This makes tornado cash compliant enough.
E.G. If the withdrawal address is under Money laundry suspicion, it may be urged to provide the origin of the transaction. That is possible [1] but there is no way of a 3rd party to Tag an account as "suspicious" based on the Tornado chain information (due to the obfuscation done by the Nodes that are getting the fees).
As far as I understand there is no accountability. The regulators would have to persecute all the nodes for helping out with the laundry. But there is no way for the nodes to know they're participating in laundry. So they cant be persecuted. Regulations needs to be invented for this kind of schema.
Please someone correct me if I said anything wrong. Im not an expert is just my conclusion based on some reading.
Check out their code on github.
But I'm on record as being in favor of full financial transparency for everybody. Every charge, every bank statement. Money, after all, is inherently social. And full transparency, while causing some problems, would eliminate a ton of others. So if you can get a legislator to submit a bill, I'll happy call them up to back it.
There are plenty of examples of that: https://github.com/jlopp/physical-bitcoin-attacks