It is without question impractical. Disposed. Best would be that the manufacturers themselves test. They should be testing anyway, to make sure that ingredient levels are correct, and that no other unwanted or harmful chemicals or additives are somehow making their way into the production stream. That's called QC, and if a company selling products for consumption aren't engaging in any QC, NONE of us should want anything to do with them. So if they are testing, as they should, there is no cause to not release the results. Simple as that.
If a manufacturer wants to refuse to test, or provide the results, then those next in the chain of liability for the product is the resellers. I sure don't blame them for testing in the absence of results from the manufacturers. Costco does this regularly with all consumables, whether tests are supplied are not. It is being responsible and caring for their customers. It's built into the price of the goods, and if you don't like it, YOU can shop somewhere else.
If a manufacturer DOES refuse to supply the results, let alone DENY their existence, thus placing the onus on the retailer, and a retailer then has it tested? It is rather disingenuous and fraudulent of the manufacturer to THEN cry foul, and provide their own, lower, results.