Well as I had it explained to me.
Run the following thought experiment.
You are a lawyer. Your client is accused of robbing a bank. The bank has the serial numbers of the bills taken. Your client pays you in cash and the serial numbers match the stolen ones.
if you agree that you are being paid by the proceeds of a crime, then congrats you agree with the law and now all you are figuring out is where the line is.
Turns out its somewhere between a forensic audit of your client and 100% certainty that you're being paid with stolen funds.
The government or prosecution claiming the funds you're being paid with are from the proceeds of crime is well within the ethical and legal bounds that lawyers are held to.
Like many issues when it comes to professionals, there is often a burden on the professional to make sure they are acting ethically. Engineers can't build a building to a clients spec if it would be dangerous. Doctors have a duty of care to not harm a patient even if the patient wants to be harmed and lawyers have a duty of care to know the origins of the funds paying them.
That's one of the burdens of being a professional
Courts are more than happy to adjudicate if there is a gray area.
Also if that doesn't convince you then remember, people can be forced to hand back assets that were stolen even if the owner had no way of knowing ti was stolen. The lawyers don't want to spend all that time defending SBF and be paid with stolen funds, just to have the courts come and tell them to pay back the money as it never belonged to SBF at all.
It's in their own best interests to know and vet where the money came from.