Usually one is bound by the laws of the district in which they stand.

So it would not be against American law for you to smoke marijuana is certain parts of Amsterdam, or wherever, as long as it is legal for you to do so according to the laws there.

But, convincing your employer that you only did it where it was legal and not here and skirting the fact that your job contract has a no pass, no pay policy or some other crap like that, it thats the case you broke the rules you employer set for you no matter where you are.

Then again, interpretation of the law, lawyers, judges, instances, and where you work can all change things majorly for the better or worse.

I dont think there is a deffinetive answer here.