Because the law against murder was broken.
> and the police can confiscate the gun and the bullets
Says who? They can only do this the law provides this mechanism. If there isn't, the gun remains the property of the owner (even if the owner is now a convicted murderer).
> but even then the government could claim to regulate an access to it - just as it could, for example, restrict access to an erupting volcano if it wanted
They could, if Congress has passed a law giving the government this regulatory power.
I get that people might feel "autonomous" is a cop-out, but legally, the argument is narrow. It isn't about people getting a way with crime. The crypto currency community would certainly be opposed to Tornado Cash relayers facing money laundering charges as well, but here they are merely saying that the sanctions authority does not extend to the sanctioning of things. And well, it either does or it doesn't. If Iran secretly plants a cherry tree in Central Park, is OFAC allowed to sanction the cherry tree, and prevent people from eating the cherries? They are arguing no, because OFAC has no authority to sanction trees.