Why travel for drug use?
It`s just as legal wherever you are as it is in The Netherlands.
Seriously though - it`s being discussed to ban non-locals from purchasing the stuff at the normal point of sale (the coffee shops).
The reason for this is the increase of crime due to non-locals traveling from all over to visit coffee shops in The Netherlands.
Although, in the larger cities (Amsterdam, Rotterdam, The Hague and Utrecht, to name a few) this is not such a large problem (yet) - along the border with Belgium and Germany they do have these problems - and that is where they started to consider the 'weed-pass'.
So, to stop this - drug addicts visiting The Netherlands should behave as if they were normal people.
No crime, no harassing others - just be quiet and enjoy whatever your drug is.