One cannot infer that because you have a drivers license, that you actually drive. There must be some other evidence, in order to make that connection.
Likewise here, one cannot infer that because you posses a medical marijuana use card, that you actually use marijuana, as the FBI has done by inserting this information into the NICS system. (<--this by inference of the AP article )
I disagree. The existence of a valid drivers license implies the holder drives. A particular licensee may not, but it is still a permissible inference.
We can also infer that someone holding a Sam's Warehouse membership card shops at Sam's. Perhaps they don't, but it is again a permissible inference. Similarly, why go to the trouble of getting a medical marijuana use card if one doesn't use marijuana? The issue, IMHO, is not whether we can draw an inference of use, but what standard of proof must there be?