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.