They mentioned that since the green tea was consumed from a long time it is clasified as food, so nobody is forced to test for side effects, dosage or effectiveness.
Other issues could be caused if the plant supplement contains also different plants, there could be similar plants that the plant collectors could slip in(by mistake or not) (I mean in general I do not know how the green tea is collected) , or the plants could be contaminated with pesticides, so I would avoid supplements if are not recommended by a doctor. As an example my son when he was young he liked carrot juice, so give it to him daily, when we went to the doctor she noticed he had is skin colored and told us to stop, I had no idea that too much carrots can be bad (it sucks that we are told that X is good for you but not that if you take too much X you will get ill)