you're missing the point of the rule. the issue with smoking weed isn't really about having impaired judgement, it's that weed is still illegal at the federal level. there are all kinds of low-level crimes that can put your clearance at risk. which kinda makes sense... they want the kind of person who strictly follows the rules for cleared positions.
I agree it's a dumb rule, but that seems orthogonal to whether Musk should have to follow an established rule. Repealing the rule would be best, but while it's in place equal enforcement is much better than enforcment for everyone except petty billionaires.
Equal enforcement just is never the way the world works, exceptions are made when its important enough.