Just because they seize their domain, doesn't mean those guys will stop selling drugs, or that their "business" will be affected for too long by a domain seizure. Obviously they need to catch them and arrest them.
Seizing the domain doesn't help at all, and the fact that they are using bitcoin is pretty irrelevant.
"Intellectual Property" is not yet one of the Horsemen of the Internet Apocalypse, but Drugs still is.