Being Libertarian doesn't necessary mean you have to be against all tax. For example (in the Libertarian point of view), taxes should only be used to maintain order, that being said the government should run prisons. However, in the case of drugs a rehabilitation program likely would be cheaper than a drug war and still under the classification of "maintaining order." Since drug users can often be violent, unable to drive, etc. and could cause harm.

Therefore, it's actually better and fair to use a small tax for rehabilitation than it is to use prisons to keep drug users locked up. This is according to the Libertarian point of view.. Strictly Libertarians are against tax, however in this case a more middle road can be argued for and be within the Libertarian's view.