Think of the MMJ card as motive. Below is the ATF letter clarifying the law back in 2011. It states "reasonable cause to believe". The fact that the buyer was honest about having a MMJ card gave the seller "reasonable cause to believe".
She screwed everyone residing in the 9th circuit with her little stunt.
You really should read the opinion. Nobody with a MMJ/Card became a prohibited person. They are still free to buy guns.