So really, legalisation would work a lot better if all countries did it (eliminating drug tourism).

No, you just need to avoid doing it in a half-hearted manner. In the case of Holland, why shouldn't there be drug tourism? Why is it any worse than, say, coffee tourism? or wine and food tourism?

Because the people drawn to drug tourism are more irresponsible, shallow, and opportunistic than your average tourist. Not always, sometimes not outrageously so, but in general. If you've been to Amsterdam, you might have noticed this, as I did.

I was surprised Amsterdam seemed to maintain a clean, moderate, respectable attitude during daylight hours. I noticed the city is an insanely wealthy and beautiful place. I would guess they are making a ton of money off tourists.

So? Lots of people are irresponsible, shallow and opportunistic. I've seen Americans complaining loudly about prices and the lack of English while touring Italy, but nobody seems to want to ban the Duomo.

  the main market for the coffee shops turns out to be drug
  tourists from abroad

That's not true. Although a number of coffee shops in Amsterdam and in cities close to the borders with Germany and Belgium primarily sell to tourists, there are many more coffeeshops that just sell to the local users.