The courts are in favor of upholding the law, not about exacting righteousness from all people.
Cobalt is a commodity which fungible and generally available on the world market. It may be refined at several steps, purchased by one company, put in storage with cobalt from other suppliers, and resold to another company. There may be general purchase records, but there are no specific records of how the physical cobalt ore was stored in a mineral silo. There is also the possibility that records that do exist could be faked for the purpose of fraud.
It is thus not common for the law to mandate that the manufacturers know exactly what has happened to every ounce of the substances that make up their product, or for the law to make this criminality contagious, such that anyone who does business with anyone who does business with anyone who does business with slave labor goes to jail.