Adding restricted ingredients to generally available supplements is a known tactic that supplement producers utilize extensively. In the past, this has been a well-known "secret" that bodybuilders basically relied on: buying tainted pre- and post-workout supplements that contain illegal steroids and such.
It's a better regulated industry now, but with the explosion in supplement popularity over the last decade I doubt there is an easy way to test and punish all manufacturers. If you look at the list of supplements included in that warning letter, they have classic nonsensical names that Chinese companies are known for (WeFun, Genergy, etc). None are on www.amazon.com anymore, but hundreds of other supplements show up with absolutely no way of telling whether any of them are clean (e.g. "Endurance 2Nite").
It's a better regulated industry now, but with the explosion in supplement popularity over the last decade I doubt there is an easy way to test and punish all manufacturers. If you look at the list of supplements included in that warning letter, they have classic nonsensical names that Chinese companies are known for (WeFun, Genergy, etc). None are on www.amazon.com anymore, but hundreds of other supplements show up with absolutely no way of telling whether any of them are clean (e.g. "Endurance 2Nite").