If you give me a $100 bill and I give you back $100 in change in smaller bills have I committed a crime?
Is it my responsibility to get AML and KYC information from every random stranger that asks me for change? If I don't is it a crime?
That is what a mixer does: makes change.
The mixer has done nothing wrong. The mixer is not obligated to keep customer records. The mixer simply makes change.
If you exchange 500'000 USD in cash for other cash, you are not making change. You are, quite likely, helping someone keep money that is 'dirty' in some sense out of reach of the the police; and you kow that. That is what 'money laundering' is.
If, after you are long gone I find out that the $100 bill you gave me was stolen have I committed a crime and am I obligate to return the $100 bill to the injured party?
In principle not, if you received that dollar bill in good faith. If the police is after someone's money for some reason, they don't want the same baknotes, only a certain amount of money. So the question is: how much will the courts demand from each of the people who cooperated in the illegal act (including anyone who consciously provided money laundering services), to make up for the stolen money, evaded taxes, fines, damages, expenses, etc.
[ The mixer ] takes in fungible BTC and returns fungible BTC. Unless you don't think BTC are fungible?
It is not up to the bitcoiners to decide whether bitcoins are fungible or not. Fungibility of bitcoins, or of dollar bills, may be a desire of their users, but it is not a legal right nor a mathematical consequence of the protocol's design.
Bills and bitcoins are fungible only if those who receive them cannot be expected to check their origin.
Dollar bills are generally fungible because those who receive them cannot be forced to check their serial numbers against a police database of stolen cash. But, if the Bank of Smallville was robbed yesterday, then a dufflebag full of neat 100$-bill bundles, with "Bank of Smallville" printed on the paper bands, will not be fungible at all. A car dealer who accepts a few bundles out of that bag could be charged with knowingly receiving stolen money; and that charge may stick in court.
Ditto if a bank receives for deposit a large pile of loose 100$ banknotes, since the bank
can be expected to check at least some of their serial numbers against said database.
If thousands of BTC are stolen by a hacker, any address that receives them, pure or mixed, will be suspected of being controlled by the thief, or by his accomplces, or by other people who may be trying to hide other money from the police. Therefore, anyone who accepts a large BTC payment and does not make a minimal effort to check their history may get charged with etc. etc.. That check may be rather impactical at present, but if the police were to set up a database of addresses containing 'dirty' bitcoins, updated in real time as they move in the blockchain, then people may be legally required to check that database when receiving a large bitcoin payment. (Note that one must be connected to the internet when using bitcoin, but not when using cash.) If that comes to pass, bitcoins will not be fungible at all.