What gives you that impression?
I've actually been pretty open about how this works, in previous posts. In my experience of this kind of scene, dds will not go anywhere near college parties. They do not make door to door deliveries, let alone hang out with their customers. Someone has to go meet them, or drive around the block to make the exchange. Also, I'm not the authority on these things, but it seems like common sense to me that they want to increase quantity of sales, but not necessarily clients. By minimizing the amount of people they are willing to do business with directly, people like JR (for example) may become a connection (through a previous connection) for all of their friends and any increased risk falls on them not the dd.
Anyway, my only point was that - as Akh has posted - I think some of the theories about possible dd involvement have taken on a kind of hollywood imagination that to me is far less plausible than certain other scenarios that may have taken place that night.
MOO