It means that the perception is that people get on marijuana, they get hooked and become "potheads," and it begins to dominate their lives. This unquestionably happens in some cases. But it also happens in the case of alcohol--and alcohol is perfectly legal.
In order to fight this argument for prohibition, legalization advocates need to make the argument that marijuana is not as addictive as government sources claim.Which now in 2010 it turns out Marijuana is actually not addictive.
One of the biggest things that effected the legal status of Hemp and Marijuana in the early days had to do with a billion dollar business not wanting to lost their number one status.