1. People with high IQs are more likely to score high on personality scales of openness to experience. They may be more willing to experiment and seek out novel experiences. Another factor could be that the messages used to attempt to deter teens from drug use weren't exactly known for the subtlety of their reasoning, so they may not have targeted the smarter group well.
2. This is due in part to the fact that money itself can buy protection against the types of criminal involvement and disease that can affect poor drug users.