It is an issue for clearance. You can get by with admitting to past youthful indiscretions during the background investigation but you absolutely can't actively violate federal law and keep a security clearance.
At best, it's uncharted territory. Someone will have to lose their security clearance for using marijuana, then they will sue the government, then the court will decide whether or not state laws can override federal laws here.
The fact that the federal government has not pressed the issue is telling as to which way they think courts will rule.
Clearance is about being compromised. Do you think Musk would reveal state secrets in exchange for weed? Do you think he would exchange state secrets to avoid the public finding out he smokes weed?
The agent who interviewed me said, "Are you the kind of person who speeds and owns up to it, or are you the kind of person who speeds and lies about it?" One of those is risky but generally acceptable.
I dunno. We're in a weird time. I can point to a few people that, i think, should not have clearances. But compartmentalization is a thing, perhaps those folks don't have a need to know the kinds of things i worry about.
My super casual, outside observer view is, the military might provide some facts and figures about payloads, but they're not saying this telescope can resolve .1mm from geosynchronous orbit. they're saying, it weighs this much and can withstand this much acceleration. There will likely be new revised policies on what's shared with spaceX about their payloads. But does anyone think Musk actions would lead him to be compromised?
It's also about the ability to follow rules (arbitrary or not). If you can't be trusted to follow the rules about not doing drugs, why should they assume you can be trusted to follow the rules about protecting classified information?
(That represents my understanding of the clearance processes' view of the issue, not my personal beliefs).
My, very shallow, understanding is, people can’t follow rules. There are a few rare individuals that can, but not the tens of thousands that are required.
Having once held a U.S. security clearance, I can guarantee you that Defense Intelligence Agency investigators are going to have a word or two with Musk. Occasional "experimental" marijuana use prior to the clearance issuance is often waived, but any use after can be a bit of a problem.