I think the problem is not the tourists.. i really don't have much knowledge about the situation really but while i was there, 2 years ago, i noticed a couple of things.
First, there were dealers on the streets, and i'm guessing it was not MJ they were selling. That gives a bad vibe to that area.. this was in one of the touristy streets, i don't know the name by heart.
Second, the red light district also had a bad vibe with the girls in the windows. The problem wasn't exactly the girls, it was the atmosphere of the place, much darker streets, sometimes dirty and an overall unclean feeling, unlike the rest of amsterdam.
I did not notice this near the coffeeshops, or with the people who were outside the coffeeshops.
So, in my opinion, the whole cannabis/coffeshops/wietpas situation, is being thrown at tourists as the ones to blame.. and maybe, there are many tourists who have a general shitty attitude and cause problems where they are, talk very loud, drink a lot, stuff like that.. and i am sure some stupid young adults eat shrooms and don't respect where they are, who's around or what time it is.
I remember, being in my hostel room with my gf, and i heard people shouting outside, they were obviously intoxicated with something (alcohol, cannabis, shrooms or something else, i really don't know.. ), so i understand the tourist argument.
It is just to flimsy for me to wrap my head around that. Clean the streets, ask the dealers to get the fuck out because honestly they don't even pretend they are doing something, they just stand by the doorways and smoke splifs. Don't allow for 30 coffeeshops in the same street. Have police doing rounds at night in those areas so people remember to behave.
And finally, educate the tourists how to act.
Amsterdam won't stop being the way it is just because some government decided to pass some law. The attitude of the city towards drugs goes back centuries, it has gone through times much more rigid than now..
And about the people who go to holland just to get dope, well.. my take is that will keep happening even if there is a wietpas. It is silly to believe it will stop. There needs to be other measures to the issue, because clearly this one will just help put a lot of people out of business at once and just harm a lot of inocent people who made their lives around the tourists with hostels, restaurants, bike renting, coffeeshops, etc that will suffer because there is a segment of people who will stop visiting Amsterdam, and most of this segment will be abiding citizens, not outlaws, dealers and troublemakers.
tl;dr: this is bullshit, the problem is not the tourists. Its the people who go there just to get some and don't even stay more than 3hours in the place where they go to buy. Other measures should be enforced.