I think a lot of them don't go to Bali specifically to use drugs. They go to Bali because of its proximity to Australia, its cost and the fact that it is a great place to party for young people.
You can't deny its appeal. You can go to the Gold Coast, pay $150 per night for an average apartment pay $8 to $10 per drink and not be guaranteed of a party.
OR
You go to Bali, stay in a 4 star hotel for $70 per night, buy cheap alcohol, be in a foreign country, wear what you want and be guaranteed a party.
I think the drug issue comes into play more when they actually get there and realise how accessible drugs are. Remember 99.9999% of tourists that go over there don't get caught using recreational drugs (it's the traffickers that do). And by the time they get there, people will often school them on how to pay off the police if you get caught.
I am not saying it is right and that young people should do it, but I am more trying to explain the appeal of going to Asia as opposed to the Gold Coast.
I've lived the past 7 years in Asia and while i am now basically a non drinker, I remember how much i loved it when i first came over.