I think the main issue is he smoked pot while he already had a security clearance, so it comes off as him acting like federal laws don't apply to him. You're right about the past history stuff, I worked in cleared environments for years, and one of my co-workers who partied a ton in high school and college answered "all of them" when the federal investigator asked which illegal drugs he'd used prior to employment. They told him that as long as he understood that he couldn't use them after he had a clearance, his past drug use wasn't an issue.