If it's flagged as a banned performance enhancing chemical, the responsibility is on Jon Jones. How or who or where or when is totally fucking irrelevant.
The responsibility is on Jones to report everything he's taking, on that I agree with you.
But if it can be proven that he is not knowingly taking the substance for performance-enhancing reasons, it is absolutely relevant.
I imagine that, depending on the amount of the substance that was found in his system, it can be proven whether or not he was taking it for said reasons.
Anderson Silva tested positive for an anabolic steroid and had a shitty excuse as to how it got into his system.
Jon Jones tested positive for an estrogen blocker (NOT a steroid and NOT a PED) and we still have yet to hear his explanation as to how it got into his system, so at the very least the jury should still be out.