I'm guessing that the issue is not so much the legality but the fact that you are a drug user. If you were on a plane and the pilot had just failed a drug test, would you feel happier knowing that he had smoked the weed in Amsterdam rather than London?
From the Royal College of Psychiatrists:
"Some users may experience psychotic symptoms with hallucinations and delusions lasting a few hours, which can be very unpleasant. Even though these unpleasant effects do not last long, since the drug can stay in the system for some weeks, the effect can be more long-lasting than users realise.
Long-term use can have a depressant effect and reduce motivation. Some researchers also suggest that long-term use can lead to irreversible, but minor cognitive deficits"
Probably not the best situation for someone trusted with making life and death decisions about patient care.