Well maybe you could consider it as an entertainment value tax. At the end of the day the cheaper it is the more people are likely to spend their money getting stoned as opposed to other forms of entertainment. It also would do more to encourage lazy stoners to get a job.
Sales tax and cigarette tax are two different taxes. To say that it should be harder to do one legal thing than to do another legal thing just because of some nebulous political ethics, in my opinion, is not in the spirit of a free country.