That's the heart of touristy downtown. Lot of guys selling fairly aggressively there towards tourists. Been offered quite a lot, I didn't have the aggression experience you seemed to experience though. That is unfortunate.
I experienced something similar at a famous parisian landmark with a person trying to sell souveniers. When I declined he got strangely aggressive and I felt like quickly getting away even though the place was packed with tourists. In Istanbul there was a man selling flowers getting a bit agressive when I declined. So like said elsewhere in this thread, this phenomenon is not drug specific.
