The reasoning behind such an imagined prohibition against a pre-existing and inalienable right, is that the person who is using marijuana is presumed insane due to the use of the drug. Under the fifth amendment, the INDIVIDUAL is protected from being denied life liberty or property without due process of law.
Also, the second Amendment is not considered a collective right, but an individual one. This has been reflected in case law at the supreme court level.
I could have my cases confused, but I believe that it was Heller vs DC that set precedent of this being a collective, or an individual right.
If, they move forward with the presumption that the user is incapable of exercising his or her second amendment rights only by use of the drug, then it will be illogical to argue against the right as being a collective one.