Because intentions matter. I will grant you that things like the tainted supplement excuse are wearing thin. But there is a difference between doing something on purpose, and doing something accidentally, and something being done to you. Obviously there is no test for intentions, but the amount of punishment can be on a sliding scale if it seems that you were somehow tricked into it or it was accidental.
Just to give a scenario. You are driving your car and a nail on the road causes a flat which causes you to lose control and hit and kill someones kid. Versus you are driving your car too fast or while intoxicated and hit someones kid. Versus you are driving your car and you deliberately run over someones kid. In each case a terrible tragedy happened, a child was killed. But are they all morally the same, and do they all deserve the same punishment?
So yes, you are ultimately responsible for everything you put into your body, and to help with that, USADA will test your supplements and has a list of approved ones. But if some shady individual gave you something and lied about what it was, or snuck it into your protein shake that is not the same thing as conspiring with a drug dealer or shady doctor and doing a cycle.