It's funny about the drug stereotype. I think it comes from the link between dreads and rastas, and rastas and weed. The only difference between rastas smoking weed and other people smoking weed is that because Rastafari is a lifestyle, it counts as a label or a category, so you can judge people belonging to that category as people who smoke weed. Whereas the rest of the weed-smokers are people who fit into so many different categories that you can't really judge them.
Anyway, it's stupid. Dreads have been around in all sorts of cultures, and even if it's part of African culture, I wouldn't see the big deal. Racists don't want to adopt other cultures, they want to eradicate them. Embracing another person's culture isn't something to be offended by in my opinion. I love other cultures and I'd love to embrace different aspects of them. I can't see how it's a bad thing to want to share culture with others. Even so, discriminating against people with dreads is just one more stupid and judgmental thing on a long list of things.