Only the police and military are allowed to sell illegal drugs here. They don't actually have moral reasons for discouraging dealers, they just hate the competition.
There are drugs everywhere, but only people with nothing to steal can get away with using them. Gringos will be hunted down like dogs even on suspicion, while dirt poor dominican crackheads make minimal efforts to hide what they are doing.
Another reason it's extremely dangerous for gringos to use drugs here is that the DR catches alot of flack for being a major gateway for South American drugs heading to the US. So the local government will happily arrest any foreigner (they don't distinguish between nationalities for this purpose, if you don't speak spanish you are a gringo whether American, French, German or Canadian) just to rub it in the US's face that the "war on drugs" seems very hypocritical given that Americans abroad always seem to be wanting to get high.
The locals that will be turning you in won't be thinking that deeply though, they only see you as a chance to do some blackmailing. Everybody loves blackmail down here. I've heard of people having to make regular blackmail payments to the very people who sold them the drugs in the first place, on threat of being turned in.
Most local dealers are cousins of police coronels or one of the many new jobless generals and as such are untouchable, but their customers are not.
Oh, and you are not "innocent until proven guilty" here either. If you are arrested you will be responsible to prove your innocence. If they decide to come and arrest you and cannot find any drugs on you, they will simply plant some to avoid the embarrassment of being wrong.
So if you do come down here make every effort to keep your nose clean, pun intended.