Considering the heavy involvement of state governments in gambling, I don't see where the moral authority to make the slippery-slope argument can come from.
In my opinion, it has nothing whatsoever to do with moral authority. It has everything to do with economic power....follow the money, and all that. Besides, "suspicion" of drugs has served as an excuse to get past so many of those pesky Consitutional barriers to search and seizure and forfeiture. Of course, these days, there isn't even much of a pretense of Consitutional consideration. I mean, if the WOD, as it currently exists, was abandoned, look at all the people who would have to find legitimate work.
You don't need moral authority when you've got the guns.