I work and live in Bangkok since 2012.
1. I still [and sure as hell i am not the only one] absolutely do not know what exactly the rules pertaining to consuming cannabis are, and which level of THC is actually legal.
i also think that the politicians involved in that law have strictly no clue either and have their brushes completely tangled on the subject...
2. Ganja consumption as a tourism asset? In many European countries and others in the world , like Spain just to name one, weed is decriminalized for personal cultivation and use, and it is very easily available (many grow it in their own homes). So unless you arrive from extremely strict places where they would "Blood Eagle" you for half a spliff, people just won.t bother to come to T. for that reason.
About the fear of the return of Backpackers and other so-called low quality tourists and cheap charlies. Well, in case one hasn't noticed or got cryogenized for 20 years, Thailand is definitely not cheap anymore, at all, for the most part. Unless you plan to vacation in some remote place between two rice paddies....