The thing is, pot is legal (state level) and illegal (Congressional statute) at the same time. Alcohol is not both at once.
The feds can disarm anybody they want to. Pretext is sufficient, but Congress is behind (as in supportive of) prosecuting the people for violation of federal law.
It’s illegal at the Federal level, but we don’t take guns away from criminals unless they’ve been convicted. What’s being suggested here is that the mere fact you’ve been processed as a medical marijuana user is treated as a conviction for a marijuana crime. I don’t think that’s appropriate.