This is a weird one. It absolutely should not be haphazardly added as a rider. The 0.4 per container is also insane. But, this really was an unintended loophole of the 2018 farm bill. Most plants grow THCa, which turns into Delta-9 when heated. They were ignorant and straight up forgot to specify anything except Delta-9.

Cannabis is a bioremediator and absorbs basically every environmental toxin from the ground (pesticides, heavy metals, etc.). Extraction (for CBD and THC oil) increases the concentration of any present toxins.

The only way you know of the problem is by thoroughly testing every batch. Pesticides that are safe at low levels can get concentrated and become really problematic at high levels.

States where marijuana is legal require all of this testing, so the products are much safer. Hemp-derived THC does not require these tests. (Same is true for CBD, but that's a while other conversation...)