Current drug tests detect any marijuana use within the past 30 days; how much does the joint you shared three weekends ago have to do with your ability to do a job today?
It shows you’re irresponsible. Kinda like what does it matter to a bank if I want to work there and have a recent bankruptcy.
An employer is free to believe that, and act on that belief. I think it's at least as irresponsible to speed - but you don't hear of employers screening out applicants with speeding tickets. If employers are restricting the pool of eligible applicants with tests that in some cases have little relationship with job performance, they should reconsider those.
I’d say illegal drug use is a bit harsher than traffic violations and is a good judgement of character.