Before I begin, keep in mind that, like every US citizen, these defendants are presumed innocent. Furthermore, the government has not disclosed any evidence that they did anything to break the law. For purposes of this post, I am assuming the government can prove money laundering as they claim
There is a well established principle of law in the U.S. known as selective prosecution.
In jurisprudence, selective prosecution is a procedural defense in which defendants argue that they should not be held criminally liable for breaking the law, as the criminal justice system discriminated against them by choosing to prosecute.
Regarding Samourai, criminal indictments were brought against these defendants. They can argue that:
persons of different age, race, religion, sex, gender, or political alignment, were engaged in the same illegal acts for which the defendant is being tried yet were not prosecuted, and that the defendant is being prosecuted specifically because of a bias as to that class.
The bias here is being a bitcoin developer, as opposed to being a banker. The latter seem immune to criminal prosecution.
There are many instances of banks being charged with crimes, yet no criminal charges are brought against individuals in those companies. Here's a nice example:
Here's one specifically regarding money laundering:
This one is a classic:
Why wasn't Goldman Sachs' CEO indicted?
I guess if big banks do it, it's not criminal?